Mike is now feeling the first flutter of sexual attraction. His hypothalamus--the brain region that triggers the chemicals responsible for emotion--tells his body to send out attraction signals: His pupils dilate; his heart pumps harder so that his face flushes; he sweats slightly, which gives his skin a warm glow; glands in his scalp release oil to create extra shine. By night's end, he gets her phone number. The next day, memories of Sue direct his brain to secrete increasing levels of dopamine, creating feelings of yearning that propel him toward the phone. He calls. She sounds excited. The dopamine released in the base of the forebrain prompts the first strong feelings of pleasure that Mike associates with Sue.

When they meet the next night at a restaurant, his stomach does flip-flops and he starts feeling giddy at the sight of her. He can think of nothing but that face, those eyes, that smile, as his brain pathways become intoxicated with elevated levels of dopamine, norepinephrine (another neurotransmitter) and, particularly, phenylethylamine (PEA). This cocktail of natural chemicals gives Mike a slight buzz, as if he had taken a very low dose of amphetamines (or a large dose of chocolate, another source of PEA). This contributes to the almost irrational feelings of attraction--we've all felt them--that begin dominating his thoughts at work, while he drives, as he goes to sleep. "It's a natural high," says Anthony Walsh, professor of criminology at Boise State University and author of The Science of Love: Understanding Love and Its Effects on Mind and Body. "Your pupils dilate, your heart pumps, you sweat--it's the same reaction you'd have if you were afraid or angry. It's the fight-or-flight mechanism, except you don't want to fight or flee."

In the weeks that follow, Mike and Sue's relationship deepens. The first night Mike brings Sue home, he dims the lights and plays a little soft music. The chemical oxytocin floods his body. Twenty years ago, oxytocin was considered a female hormone useful only as a trigger for labor contractions and to induce lactation. In the '80s, research found that it is produced in the hypothalamus by both men and women, helping to create feelings of caring and warmth (thus bonding mother and baby after birth and during nursing). As Sue's oxytocin also surges, the couple begin forming a bond. Scientists now think that oxytocin actually strengthens the brain's receptors that produce emotions. Oxytocin increases further during touching, cuddling and other stages of sexual intimacy. It may also make it easier to evoke pleasant memories of each other while apart. Mike can think of Sue and experience, in his mind, the way she looks, feels and smells, and that will reinforce his connection to her. (Helen Fisher, a Rutgers University anthropologist, is conducting research with magnetic resonance imaging to track which parts of the brain change when someone is in love.)

Next comes the wedding. Honeymoon. Now what? Fast-forward 18 months. At this point, Mike and Sue could be at a crossroads. Science tells us that 18 months to three years after the first moment of infatuation, it's not unusual for feelings of neutrality for one's love partner to set in ("Why don't you take out the trash?" vs. "I dream about you all the time"). For many, there could be a chemical explanation. The mix of dopamine, norepinephrine and PEA is so much like a drug, say scientists, that it takes greater and greater doses to get the same buzz. So after someone has been with one person for a time, his brain stops reacting to the chemicals because it is habituated. "The brain can't maintain the revved-up status," says Walsh. "As happens with any drug, it needs more and more PEA to make the heart go pitter-patter."

Couples with attachments that are shaky for other reasons (money woes, abuse, irreconcilable differences) may part and--because the body's tolerance for PEA soon diminishes--seek someone new with whom to find the thrill of early love. More likely, however, committed couples will moveon to what science suggests is the most rewarding and enduring aspect of love. Though the same addictive rush isn't involved, ongoing physical contact, not just sex, helps produce endorphins, another brain chemical, and continued high doses of oxytocin. Endorphins calm the mind and kill anxiety. Both chemicals are like natural opiates and help stabilize the couple by inducing what famed obstetrician Michel Odent, of London's Primal Health Research Center (whose book, The Scientification of Love, will be published this year), calls "a druglike dependency."

Even in the animal world, neuroscientists had long wondered what kept prairie voles loyal to one mate while their cousins, the montane voles, were promiscuous maters. As it turned out, prairie voles are much more sensitive to the effects of oxytocin. In experiments, when those receptors are blocked, the animals' stay-at-home tendencies decline. "At present, our knowledge of neuroscience is doubling every two and a half years," says Robert Friar, professor of physiology and human sexuality at Michigan's Ferris State University. "That means that in the last two and a half years we have learned more than all prior humans about the workings of the brain." Says the University of California's Fallon: "Certainly the '90s are a blur for people in neuroscience. We all want to be up twenty-four hours a day so we don't miss a thing."

But in the end, will love's mysteries ever unravel in a laboratory? Some, like Fallon, say yes. Others, perhaps most of us lucky enough to have experienced true love, might believe--and wish--otherwise. Even in this advanced age of science, where we can transplant organs, map the human genome and clone our own offspring, we still have not come close to understanding what, exactly, ignites our spark of life, our souls, our very being. Maybe, possibly, that will remain true for the farthest reaches of love.