Hypogonadism is a major manifestation of zinc deficiency in humans and animals, but the mechanisms responsible for this hypogonadism are unclear. We evaluated the pituitary gonadal axis of zinc-deficient rats with basal testosterone, follicle-stimulating hormone, and luteinizing hormone measurements and both luteinizing hormone releasing hormone (GnRH) and human chorionic gonadotropin (HCG) stimulation tests, and we utilized pair-fed (isocalorically fed to zinc deficient), weight-restricted (fed to weigh the same as zinc deficient), and ad libitum-fed controls. Basal testosterone levels were significantly decreased in rats with zinc deficiency (0.6 +/- 0.2 ng/ml) and weight-restricted rats (0.6 +/- 0.1 ng/ml) compared with pair-fed rats (1.2 +/- 0.2 ng/ml) and ad libitum-fed rats (1.6 +/- 0.2 ng/ml, p less than 0.01). Appropriate pituitary response to GnRH were noted in each group of rats. In a final study, zinc-deficient and weight-restricted rats were given injections of 1 U HCG for 5 days. Weight-restricted rats demonstrated a highly significant increase in stimulated testosterone (7.6 +/- 0.9 ng/ml) compared with zinc-deficient rats (2.1 +/- 0.3 ng/ml, p less than 0.0005). We conclude: (1) hypogonadism occurs in both zinc-deficient and weight-restricted rats compared with pair-fed and ad libitum-fed controls; (2) pituitary reserve is intact in zinc-deficient rats; (3) HCG stimulation demonstrates Leydig cell failure in zinc-deficient but not weight-restricted hypogonadal rats; and (4) the hypogonadism in zinc-deficient rats results mainly from Leydig cell failure.