Data from a variety of different sources lead the conclusion that hallucinogenic drugs exert their critical action at a specific serotonin receptor subtype, 5-HT2. action (Depending on the particular brain area, the may be either excitatory or inhibitory.) This does not preclude in the possible involvement of other neurotransmitters the action of hallucinogenic drugs. In fact, structural differences in the drug molecules are probably responsible for variations in the phenomenological effects produced by them. A major remaining issue is how this triggering action of hallucinogenic drugs at 5-HT2 receptors evokes we as a the experience identify hallucination?that is, changes the sensory-perceptual, psychic, and somatic domains. As further progess ismade in elucidating the neural substrates of emotion, perception, and other processes, we will better understand how a change in the modulatory influence exerted by serotonin at its receptor sites throughout the central nervous system mediates hallucinogenesis. Recent anatomical studies indicate that the 5-HT2 receptors are abundantly local ized in neocortical, limbic, and brainstem sites. In addition, noninvasive scanning techniques for studying brain function in conscious humans, such as positron emission resonance tomography (pet) and nuclear magnetic (nmr), are proving invaluable to research on psychoactive drugs. Finally, we may ask what functions subserved by the other receptor sites where serotonin acts. Recent evidence indicates that one of these sites (5-HT1A) may be important in producing anxiety, while another may be critical in the development of migraines. It is important to remember that under normal conditions serotonin exerts an equal action at all of its receptor sites. or However, when one site is selectively activated blocked, whether by drugs, endogenous biological factors, or environmental toxins, it leads to a perturbation which may be manifested as anxiety, migraines, or hallucinations.