A method for the radioimmunoassay of diethylstilbestrol (DES) was established to evaluate the distribution of DES in patients with prostatic carcinoma receiving intravenous infusion of 500 mg of diethylstilbestrol diphosphate (DES-DP). The plasma DES concentration was markedly elevated to 4.9 μg/ml immediately after DES-DP infusion and rapidly decreased to 0.8 μg/ml at 3 hr after the DES-DP infusion. The DES concentration in the prostatic tissue was 1.6 μgig w.w. at 1 hr after infusion, and thereafter was predominantly similar to that in the plasma. The DES excretion in the urine was 23.7 mg at 0 to 3 hr after DES-DP infusion, and decreased gradually thereafter. In addition, the DES distribution in the subcellular fractions of benign hypertrophic prostates was studied following DES-DP infusion. The DES concentration in the nuclear fraction decreased with time. Furthermore, the protein specifically bound to DES was not detected in the cytosolic fraction. Therefore, the DES converted from DES-DP did not accumulate in the prostate. Subsequently, the bioavailabilities of DES were examined. The dihydrotestosterone (DHT) content of the prostatic tissue was significantly decreased at 24 hr after DES-DP infusion. However, the cytosolic androgen receptor content was insignificantly changed within 24 hr of infusion. Since the plasma luteinizing hormone (LH) level in the castrated patient with prostatic cancer was significantly lowered 3 hr after infusion, and further decreased with time, the main effect of DES was considered to be suppression of hypophyseal function.