IT'S Valentine's Day and lovers are on Cloud 9, but they're actually caught up in a chemical, hormonal whirlwind. This Love Potion No. 9 causes romance, passion and, if they're lucky, abiding love. Zhang Qian takes a sip.

Richard Zhu still remembers how fast his heart beat six years ago when the young woman with long hair and big, shiny eyes entered their French class. He stared, his palms became a bit sweaty. That was his first encounter with the young woman who would become his wife.

Fortunately, she felt the same way: they had chemistry.

Cupid had scored a direct hit, and the rest was history - actually, the rest was biochemistry - from the moment the dart struck, to the flowering of romance and passion, to the exchange of wedding vows. And Cupid the biochemist continues to play a role in long-term bonding, comfort and physical pleasure, even after the first torrents of passion abate for both partners.

Cupid's arrows are dipped in a potent cocktail that acts immediately and over time. The potion includes adrenaline, oxytocin, dopamine, phenyl ethylamine (PEA), norepinephrine, endorphins, vasopressin, serotonin, and, of course, testosterone.

"I cannot say exactly which part of her is most attractive, but every moment around her makes me happy," says Zhu, now a university instructor in his early 30s.

Scientists say that in Zhu's case, as in virtually every other case of early head-over-heels love, what we "love" is usually not a specific person, but the pleasurable feelings she or he generates in us. It's not the "pretty" or "handsome" face (though there's always something that makes that face appealing); it's not just the sweet voice or flashing eyes.

But something about that person gives us a high. There's body language, speed and rhythm of speech, touch, scent and pheromones. And, there's what they say (though that's less important).

"What we first 'love' is the experience in our brain," according to Mao Lihua, associate professor of psychology in Peking University. Love is addictive, and there are indeed, love junkies, who love the highs but don't commit.

Lisa Lin, a 23-year-old office worker, describes butterflies in her stomach when she went on her first date with her boyfriend. "It was sweet but I was very nervous and my heart was racing, especially when he held my hand for the first time," she says.

That beating heart, which always accompanies the first flush of early love, is an important signal of love and contributes to the universal notion that the heart is the source of love.

But the pounding heart (even heartache and broken hearts) are only a symptom - the true source of love is the brain and the cocktail of chemicals that are racing around and sending out signals of rapture to the rest of our body.

Whether it's the chemicals that cause the love or the love that causes the chemical reactions can be debated endlessly.

In any case, when we see that special person, smell his or her distinct scent, and pick up their odorless pheromones, the nerve circuits in our brain immediately pick up those signals, and demand the pituitary gland secrete hormones, which in turn trigger secretion of other chemicals. These cause the heart to beat faster, the breathing to quicken, the pulse to race, the blood flow to increase; they cause sweaty palms and dry mouth.

All these reactions are sent back to the brain as clues and, along with other information, the brain concludes it must be love.

"All the complicated processes are triggered swiftly and they work extremely fast and automatically," says psychologist Mao. "So all you can feel is that you are 'hit' - it's love at first sight."

However, a fascinating aspect is that some of the same chemical chain reactions (such as the adrenalin rush and pounding heart) take place whether one is faced with one's dream lover or a dangerous monster, says Mao, but with completely different results.

The brain comes to different conclusions based on the different situations and clues; the brain decides whether this exciting person or, say a bear, is to be loved or feared, whether one should hug or run.

Constant secretion of chemicals keeps people immersed in feelings of love; these feelings continue even when one person is rejected, so that the person's passion gets stronger until secretions finally ebb.

Some people consider it cynical to explain romantic love - and sentimentalized "eternal love" - as a matter of brain chemistry intended to further reproduction and continuation of the species.

"But the amazing heart-pounding feeling of love is undeniable," says Mao. "That's why some scientists consider love an addiction like drugs, which work by exciting the brain's reward circuit, making people feel happy and longing for the same feeling again."

But other good things in a loving relationship can trigger the reward circuit in the brain in the same way; these include intimacy, reliance, family love and love of offspring.

The powerful and stimulating love potions remain in high concentrations only for six months to four years, but many people remain attracted and committed after dozens of years of marriage.

Anthropologist Helen Fisher, together with her colleagues Bianca Acevedo and Lucy Brown, did brain scans on a couple who had been in love for decades, and found that the sight of the mate triggered the same brain reaction as new love. The same brain region that makes dopamine becomes activated when you have just fallen in love and in long-term love or attachment. Of course, this doesn't happen all the time.

Wang Fuhai, 85 years old, met his wife-to-be on a blind date when he was 20 years old. They have three children and three grandchildren. Despite the inevitable quarrels, some big and some trivial, over the years, they love walking hand in hand in the morning, reading newspapers in the afternoon and watching her favorite TV soaps, though he can never figure them out.

"Having her as my wife is the best thing in my life," says Wang. "I hope we will be together in the next life as well."

Who cares if it's brain science? It's still love.

The biochemistry of love

Here are some of the chemicals that are involved in love and its three stages: romantic feelings, physical attraction and emotional attachment.

? Adrenaline - Gives us that rush.

? Phenylethylamine (PEA) - A naturally occurring amine in the brain (also in chocolate), which is similar to an amphetamine and releases the pleasure chemicals norepinephrine and dopamine. PEA is present when one is falling in love and is responsible for the rapturous, head-over-heels feeling, the high. Dopamine plays a big part.

? Norepinephrine - A hormone neurotransmitter that causes sweaty palms and a pounding heart.

? Dopamine - A neurochemical associated with mate selection and especially with rewards and pleasure. It triggers the release of oxytocin.

? Oxytocin - The "cuddle hormone." In both men and women, oxytocin is released during touching and sex. In women it is released during labor and breast feeding. It is produced throughout a relationship.

? Testosterone - Both men and women produce it; lust is accompanied by a surge in testosterone in both sexes.

? Endorphins - Neurotransmitters that are like opiates. They calm anxiety, relieve pain and reduce stress. They are associated with feelings of comfort and attachment - lasting love and affection.

How to fall in love

Professor Arthur Arun of New York University at Stonybrook performed an experiment on attraction:

He put many sets of complete strangers together for half an hour and they revealed intimate details of their lives for half an hour.

Then they sat quietly near each other, stared deeply into each other's eyes and said nothing for four minutes.

He found that after the 34-minute experiment many of his subjects were very attracted to each other. Two later got married.