We studied the effects of oestrogen administration on the kinetics of testosterone metabolism in patients with prostatic carcinoma. We investigated changes in the metabolic clearance rate, transport and metabolic rate constants and volumes of distribution. In all patients but one, the metabolic clearance rate and volumes of distribution were low during oestrogen administration. The beneficial effect of oestrogen therapy in patients with prostatic carcinoma may be related to the increase in the rate of testosterone metabolism (K2) within a smaller volume of distribution (V1).