Serial assays of hormones and their metabolites are reported in the urine of three male and four female homosexuals. Urinary testosterone levels were abnormally low in the two men who practised exclusive homosexuality and were within the normal range in the third, who had both homosexual and heterosexual relationships. In the women assays were generally performed throughout one menstrual cycle; in three the pattern of hormone excretion was ovulatory in character, while in the fourth evidence for ovulation was equivocal. Levels of testosterone and luteinizing hormone (L.H.) were raised in the female homosexuals, while those for oestrogens, particularly oestrone, were below the range for normal heterosexual subjects during their reproductive life; readings of follicle-stimulating hormone (F.S.H.) and pregnanediol were normal in three women. The data reported here are in keeping with the view that abnormalities in endocrine function may occur in both male and female homosexuals.