Circadian dynamics of biosynthetic activity in pineal glands of adult relatively wild and domesticated silver fox females was studied beyond the reproductive season using radioimmune and fluorometric methods. The level of melatonin, the principal pineal hormone; activities of enzymes controlling its biosynthesis; the level of its precursor and one of its metabolites, as well as those of neurotransmitters involved in the regulation of biosynthesis exhibited more or less pronounced circadian changes. The concentration of melatonin in the pineal gland at night was considerably higher in domesticated foxes than in relatively wild ones. The most likely reason for the elevated concentration of melatonin at night is its slow secretion from the pineal glands of domesticated foxes, because its concentration in plasma does not differ from that in relatively wild animals. The results obtained suggest that selection for domestic behavior has affected the adrenergic mechanism of regulation of pineal gland rhythms, as well as other functions of sympathetic divisions of the central nervous system.