Whole liver homogenates from male and female rats were studied for their ability to reduce the 17,21-dihydroxy-20-ketone sidechain and Δ4-3 ketone group of cortisone. The previously reported sex difference in both sidechain and ring A reduction was confirmed. Sidechain reduction is more rapid in the male; ring A reduction more rapid in the female. Castration of the male depressed sidechain reduction and increased the rate of ring A metabolism. These changes were prevented by testosterone administration if started immediately following surgery. Ovariectomized female rats showed no change in ring A activity and only a slight increase in the rate of sidechain reduction. Estradiol to males and testosterone to females depressed the sidechain and ring A reduction respectively. Neither pregnancy nor progesterone administration significantly influenced cortisone metabolism in homogenates from female rats. Injections of cortisone acetate reduced C-20-keto reductase activity in both males and females, had no effect on Δ4-3 ketone reduction in females but sharply depressed this function in males. Young rats from 5 to 75 days of age were studied at various intervals to determine the manner in which the sex difference in cortisone metabolism develops and how this is modified by castration.