The discovery of several endogenous substances with morphine-like activity (endorphins and enkephalins) which possess potent behavioral effects, interfering with food and water intake, has led to suggest their implications in the pathogenesis of human obesity. This suggestion is mainly based on: 1) the ability of opiate antagonists naloxone and naltrexone to reduce food intake in some particular situations associated with obesity: 2) the existence of raised plasma levels of beta-endorphin in obese children and adults not corrected by weight loss; and 3) the increased responsiveness to the metabolic and hormonal effects of opiate agonism and antagonism found in obese but not in normal weight subjects. Although the problem still awaits a definite answer, it seems not hazardous to hypothesize a role for beta-endorphin in some pathogenetic events associated with human obesity.