The intraventricular injection of D-alanine-methionine-enkephalinamide (D-Ala2-Met-enkephalinamide), a synthetic analog of Met-enkephalin that is resistant to enzymatic degradation, inhibits copulatory behavior in sexually vigorous male rats in doses which do not influence motor activity or feeding behavior. This effect is prevented by naloxone, a specific inhibitor of opioid receptors. In addition, injections of naloxone induce copulatory behavior in sexually inactive male rats. These results suggest that endorphins play an important role in the regulation of sexual behavior.