The antinociceptive properties of intrathecally (i.t.) administered [D-Ala2, N-methyl-Phe4, Gly5-ol]enkephalin (DAMPGO) and [D-Pen2, D-Pen5]enkephalin (DPDPE), selective opioid agonists for mu (mu) and delta (delta) sites, respectively, were compared in rats. DAMPGO and DPDPE elevated tail-flick latency (TFL) in a dose-dependent manner, and the spinal antinociceptive actions of both drugs were reversed by the opiate antagonist naloxone. These findings suggest that both DAMPGO and DPDPE interact with spinal opiate receptors to elevate TFL. Another set of experiments was done to determine the involvement of local spinal serotonin (5-HT) or norepinephrine (NE) in DAMPGO and DPDPE-induced spinal analgesia. Both the alpha 1 noradrenergic receptor antagonist WB-4101 and the alpha 2 blocker yohimbine failed to alter the antinociceptive actions of DAMPGO and DPDPE. Similarly, the 5-HT receptor antagonists pindolol, ritanserin and ICS 205-930 (selective for 5-HT1, 5-HT2 and 5-HT3 sites, respectively) failed to inhibit opioid-induced spinal analgesia. Thus, while DAMPGO and DPDPE produce antinociception via an interaction with spinal opioid receptors, apparently neither drug activates endogenous monoaminergic systems.