Naive laboratory rats, without pre-exposure to operant training procedures or to opioids, were shown to self-administer directly into their cerebral ventricles the endogenous opiate peptide, leucine-enkephalin. They were shown to self-administer the peptide consistently for six consecutive days with no indication of the development of tolerance. The results indicate that leucine-enkephalin may possess potent reinforcing properties and suggests that it may play a role as an endogenous reward transmitter.