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Melatonin, the principal hormone of the pineal gland, has several biological effects’. The hormone regulates the repwlustive alterations that occur in response to chaqges in day length in seasonally breeding mammals!. Melatonin can also alter the timing of mammalian circadian rhythms, and appears to function in concert with light to hold circadian rhythms in phase with prevailing environmental conditions’, Melatonin inhibits the Ca”*-dependent release of dopamine in mammalian retinad, and appears to act in verte brate retina to affect several lightdependent functions, including shedding of photopigment discs and phagocytosis? The pineal hormone can also induce sleep in humans; this hypnotic effect may be separated from its effects on circadian rhythm@. Melatonin appears to exert these effects through pharmacologically specific, high-affinity receptor+? Recent progress in this area has been the cloning of a …