Historically, the development of biological science has proceeded from the anatomy to the physiology of particular systems. Knowledge about neurotransmitters in the brain has evolved from the discovery of the compounds per set through the elucidation of their biosynthetic and degradative pathways, and finally to the current focus, the regulation of these processes. Regulation in a biological system implies the capacity to respond to perturbations. The introduction of standard and speci­ fiable disturbances in a biological system followed by examination of the mechanisms involved in response, i.e. that dampen and/or cancel the induced changes, has proved to be a successful way to elucidate regulatory mechanisms. Psychotropic drugs have been most useful to students of regula­ tory mechanisms in that calvarial prisoner, the brain, for they allow prospective manipulation of variables like dosage and time. Those of us interested in the biological substrata of behavior are especially excited that disturbance induced by drugs leaves the organism intact and behaving, and we can observe behavioral correlates. The pharmacological strategy is, of course, not unique to brain scientists. The relationship between psychotropic drugs and our emerging knowledge of neurotransmitter regulation is, for example, akin to the development of anticancer agents in company with the uncovering of many mysteries of molecular biology.