Intraventricular administration of the enkephalin analog, [D-Ala2]-metenkephalin, induces profound and long-lasting analgesia, as well as other opiate-like behavioral effects in the rat. This analgesia was highly dose dependent, of much greater magnitude, and about 30 times longer lasting than that produced by the naturally occurring peptide, methionine-enkephalin. The behavioral effects of the [D-Ala2] analog could be completely reversed by the opiate antagonist, naloxone, suggesting that these effects were mediated by opiate receptors. Systemic administration of naloxone alone resulted in a significant increase in pain sensitivity. These findings support the view that endogenous opiate systems may play an important role in modulating pain sensitivity.