Effects of estrogen treatment on testicular human chorionic gonadotropin (hCG) and follicle-stimulating hormone (FSH) receptors in man were investigated. Ten patients (aged 58 to 75, mean 67 years) with prostatic cancer were treated with diethylstilbestrol diphosphate (DESDP) (300 mg daily, oral administration). They were divided into two groups: 5 of them (aged 58 to 74, mean 67 years) were orchiectomized after 7 days of treatment and the remainder (aged 60-75, mean 67 years) after 6 months of treatment. hCG and FSH receptors in the resected testes of each group were measured and compared with those of age-matched prostatic cancer patients without any treatment (controls). After 7 days and 6 months of DESDP treatment, the number of hCG receptors decreased to approximately 53% and 24%, respectively, of that of the controls. FSH receptors in the testes of the patients treated with DESDP did not differ significantly from those of the controls.