Melatonin is an indolamine synthesized from tryptophan in the pineal gland. It is regarded as "the epiphyseal hormone". Its antigonadotropic action has been demonstrated in animals, both in vitro and in vivo, together with its inhibitory effect on numerous endocrine functions and its anti-convulsive properties. Recently developed assay methods have made it possible to obtain clinical data, for the moment purely descriptive. Melatoninis present in several body fluids, such as urine, blood and cerebrospinal fluid. It is secreted in circadian cycles, with low concentrations during the day and high concentrations at night; sex has no influence on this pattern, but secretion is highest in the summer and winter and lowest in the spring and autumn. The part played by melatonin in the genesis of puberty is undetermined. Melatonin secretion appears to be mediated by the adrenergic system, since beta-blockers inhibit the nocturnal rise. However, contrary to what happens in animals, most beta-adrenergic stimulants do not increase melatonin concentrations. Abnormal concentrations or perturbations in the melatonin secretion rhythm have been demonstrated in such diseases as breast cancer, cirrhosis of the liver, Klinefelter's syndrome, Cushing's syndrome and haemochromatosis. Depressive syndromes are often associated with abnormal melatonin cycles. It has been suggested that melatonin could be used as a biological marker in cancer and psychiatric diseases, but its physiological function in man remains obscure.