MUCH OF the biochemical basis of risk-taking is thought to be a function of the limbic system -- a relatively primitive area of the brain involved in the regulation of human drives and emotions. It is here, for example, that special receptor cells respond to endorphins, the opiate-like pain killers produced in the brain. And it is here that the blood-rich hypothalamus gives rise to feelings of hunger and thirst, reward and punishment, sexual arousal and anger.

In particular, researchers have found links between certain behavioral patterns and a family of limbic brain chemicals called monoamines. The monoamine epinephrine, for example, is secreted by the adrenal gland and is involved in rapid physiological response to emergencies -- the "fight or flight" mechanism.

Norepinephrine, epinephrine's chemical cousin, is another monoamine. Shortages of it have been implicated in clinical depression, while mania may be the result of norepinephrine excess.

Other behaviorally significant monoamines include serotonin, a potent stimulator of cerebral activity, and dopamine -- increased levels of which have been tied to schizophrenia. Dopamine is also thought to be an important ingredient in the experience of pleasure. Amphetamines, which facilitate dopamine's effects, increase the rate at which rats seek stimulation.

Critical to the regulation of these influential chemicals is an enzyme called monoamine oxidase, which breaks down neurochemical transmitters like epinephrine. Another enzyme, dopamine beta hydroxylase (DBH) further complicates the regulatory scheme by converting dopamine into norepinephrine.

Research into the specific behavioral roles that these chemicals play is hindered in part because brain chemicals are difficult to measure directly. Moreover, the sensitivity and level of functioning of the monoamine system probably varies among individuals, making such measurements difficult to interpret. In addition, scientists suspect, a single chemical may have entirely different effects in different parts of the brain.