Serum pituitary levels of growth hormone (GH), thyrotropin (TSH), prolactin (PRL), luteinizing hormone (LH) and follicle stimulating hormone (FSH) were measured in sexually mature (adult) and sexually immature (juvenile) male rats who had been deprived of dietary zinc for 15 and 7 weeks, respectively. When compared to pair-fed control rats receiving a zinc supplemented diet, both the adult and juvenile zinc deficient rats had significantly lower body weights, tail lengths and ventral prostate weights. The testes of the sexually immature rats were also smaller than those of the pair-fed animals. In sexually mature, zinc deficient rats serum concentrations of GH and testosterone were significantly lower and serum LH levels significantly higher than in ad libitum fed control rats. Pituitary and hypothalamic levels of other hormones did not differ from values recorded in control animals. In sexually immature zinc deficient rats serum concentrations of GH were also significantly depressed; pituitary content and concentration of LH and pituitary and serum levels of FSH were significantly increased over control values. No discernible effects of zinc deficiency upon hyplthalamic content of LH-releasing hormone or serum concentrations of PRL or TSH were recorded in juvenile rats. Zinc deficiency has minimal effects upon the hypothalamic-pituitary axis of sexually mature rats. In sexually immature males, zinc deprivation leads to impairment of gonadal growth and increased synthesis and/or secretion of the pituitary gonadotropins.