The effect of epithalamin on circadian rhythms of thymic serum factor titers and melatonin concentrations in the blood of elderly people was studied. Course treatment with epithalamin modulated the rhythm of the thymic endocrine function. The increase in the titer of thymic serum factor at night coincided with the increase in blood melatonin concentration and shift of its acrophase to 3.00, which was characteristic of young people. In elderly people with preserved nocturnal peak of the thymic serum factor titer before therapy the nocturnal (3.00) concentration of melatonin was high and did not change after drug therapy. The correlation between melatonin concentration and titer of thymic serum factor increased after epithalamin treatment.