Opiate peptides and opiate receptors are concentrated in parts of the limbic system and striatal complex. It has been suggested that endogenous opiates may be involved in sexual behavior. This possibility was tested by observing the effects of the opiate antagonist naltrexone on the mating behavior of male hamsters. A detailed ethological analysis revealed several aberrations in mating behavior of naltrexone-treated animals. These deficits were more severe in animals receiving 20.0 mg/kg of naltrexone than in animals receiving 2.0 mg/kg. In general, their behavior appeared sluggish and inattentive. They were capable, however, of successfully impregnating females. Animals receiving 200.0 mg/kg naltrexone showed severely impaired copulatory behavior, but did not appear distressed, ataxic, or somnolent. In a second experiment animals were sacrificed at various stages of mating behavior and their brains assayed for opiate receptor activity; no differences were found. In a third experiment, continuous administration of naltrexone by slow release pellets decreased fertility in female hamsters and survivorship of their pups.