1. The antinociceptive effects of the delta opioid receptor selective agonist, DPDPE [(D-Pen2,D-Pen5)-enkephalin] was studied in rats aged postnatal day (P) 14, P21, P28 and P56. 2. Antinociceptive effects of DPDPE were measured as percentage inhibition of the C-fibre evoked response and post-discharge of dorsal horn neurones evoked by peripheral electrical stimulation. DPDPE was administered by topical application, akin to intrathecal injection. 3. DPDPE (0.1-100 microg) produced dose-related inhibitions at all ages; these inhibitions were reversed by 5 microg of the opioid antagonist naloxone. 4. The dose-response curves for C-fibre evoked response and post-discharge of the neurones were not different in rats aged P14 and P21. DPDPE was significantly more potent at P14 and P21 compared with its inhibitory effects on these responses at P28 and P56. 5. DPDPE produced minor inhibitions of the A-fibre evoked response of the neurones at P14, P21, P28 and P56, suggesting that the inhibitory effects of DPDPE are mediated via presynaptic receptors on the terminals of C-fibre afferents. 6. Since spinal delta opioid receptor density changes little over this period, the increased antinociceptive potency of DPDPE in the rat pups compared with the adult is likely to be due to post-receptor events, or in developmental changes in the actions of other transmitter/receptor systems within the spinal cord.