Six prostatic cancer patients were treated with a gonadotropin-releasing hormone agonist analog (ICI 118630, ICI Pharmaceutical Division, Macclesfield, England) for 4 weeks, and serum testosterone (T) had fallen to the castration level in all patients. Subsequently, three of the patients were orchiectomized. There were no significant differences in follicle-stimulating hormone (FSH) and human chorionic gonadotropin (hCG) receptors between the testes treated with ICI 118630 and testes obtained from untreated prostatic cancer patients. In the other three patients, testicular responsiveness evaluated using hCG stimulation was similar before and after 4 weeks of treatment. In vitro studies testes removed from untreated prostatic cancer patients were cultured in a medium with or without ICI 118630 for 5 days. There were no significant differences in FSH and hCG receptors and T production between the two groups. In conclusion, ICI 118630 had no direct inhibitory effect on human testes.