Adult male rats received daily injections (sc) of gonadotropin releasing hormone antagonist (0.2 mg/kg(-1) x day(-1)) for 21 days when they were sacrificed on day 22, adrenal weight, adrenal A5-3beta (delta 5-3beta) hydroxysteroid dehydrogenase (Delta5-3beta-HSD) activity and serum level of corticosterone were increased significantly while testicular 17beta (17beta) hydroxysteroid dehydrogenase (17beta-HSD) activity and serum level of testosterone and spermatogenesis were decreased in the rats fed on 5% casein diet. GnRH antagonist treated rats fed on 20% casein diet, resulted significant decrease in adrenal weight, serum corticosterone and adrenal A5-3beta-HSD activity while testicular 17beta-HSD activity serum testosterone levels and the weights of sex organs were increased with respect to anti GnRH treated rats fed on 5% casein diet. But the GnRH antagonist treated rats fed on 20% casein diet showed decreased spermatogenesis quantitatively and sperm count appeared similar to anti GnRH treated rats fed on 5% casein diet. These results indicate that high casein diet protects adrenocortical activity and stimulates testosterone synthesis without effecting spermatogenic arrest in GnRH antagonist treated rats. It may be concluded that GnRH antagonist in presence of high milk protein diet may be considered to be a suitable antihormone in the development of an ideal male contraceptive.