High affinity, stereospecific recognition sites (receptors) for putative neurotransmitters, neuromodulators, and many psychotherapeutic agents have been identified in the mammalian central nervous system. It is currently believed that the interaction of a neurotransmitter, neuromodulator, or psychotherapeutic agent with these sites initiates a series of events resulting in either a physiologic/behavioral response (in the case of neurotransmitters and neuromodulators) or a pharmacologic action (in the case of a psychotherapeutic agent). Furthermore, the high affinity, stereoselective binding of psychotherapeutic agents to brain also suggests that previously undescribed endogenous modulators, which physiologically mimic (or antagonize) the actions of these agents may be present in brain. Several receptor systems are currently under study, including: a) benzodiazepine receptors (believed to play a role in the pathophysiology of anxiety, seizure disorders, musculoskeletal disorders, and sleep disorders; b) adenosine receptors (believed to be involved in sleep/wakefulness); c) receptors for tricyclic antidepressants; d) receptors for central stimulants (e.g., amphetamine); e) receptors for hallucinogens (e.g. phencyclidine).