The response of 3H-testosterone and 3H-dihydrotestosterone to the administration of cyproterone acetate (CA) over a period of 5 days was investigated. Both tracers were injected intravenously in patients with benign prostatic hypertrophy. Blood was withdrawn for up to 5 h. Benign prostatic hypertrophy tissue was obtained by transurethral resection. Nine patients served as controls. Eleven patients received 300 mg CA intramuscularly. Cyproterone acetate suppressed testosterone and FSH, but not LH. 3H-testosterone was cleared more rapidly from plasma in the patients given CA presumably due to increased metabolism in the liver. 3H-dihydrotestosterone, however, remained virtually uninfluenced. Moreover, CA did not significantly alter the 3H-testosterone and 3H-dihydrotestosterone uptake and metabolism within prostatic tissue.