The endogenous opiate system is involved in the regulation of numerous bodily functions, but the literature suggests that the effects of endogenous opioids differ among species and between animals and man. Naltrexone, a relatively pure opiate antagonist, appears to have significant effects on the secretion of the gonadotropins (luteinizing hormone and follicle-stimulating hormone), adrenocorticotropin (ACTH), cortisol, and probably catecholamines. Naltrexone appears to have minor or no effects on prolactin, the pituitary-thyroid axis, growth hormone, insulin, glucagon, vasopressin, and the gut hormones. Naltrexone also seems to reduce food intake and cause weight loss in humans. The dosages of opiate antagonist and the presence of other variables play a major role in the responses seen in various studies.