The effect of endogenous and exogenously administered oestrogens, androgens and progesterone on plasma and urinary uric acid and uric acid clearance was studied in a total of 65 healthy volunteers, including normal menstruating and post-menopausal women, girls with primary amenorrhoea and adult male subjects. A serial study throughout a full cycle in 3 women showed an inverse relationship between plasma uric acid levels and endogenous oestrogens. Administration of conjugated and synthetic oestrogens produced a fall in plasma uric acid concentration through a uricosuric effect in most subjects of both sexes. Testosterone propionate caused a definite increase in plasma uric acid levels in post-menopausal women while endogenous testosterone changes due to Leydig cell stimulation produced no definite effect in male subjects. Administration of a progesterone preparation produced an effect similar to that of oestrogens in post-menopausal women. The evidence presented here supports the view that sex steroids play a significant part in uric acid regulation in biological fluids of both sexes.