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In this study, we have used a variety of in vitro and in vivo techniques to demonstrate the presence, and examine the function, of [3H]U69593 binding sites in the spinal cord of the 9-16-day-old rat in comparison to the adult. Equilibrium binding of [3H]U69593 to homogenates of adult rat spinal cord revealed a single population of non-interacting sites with a maximum binding capacity of 10.4 +/- 1.4 fmol/mg protein and an apparent equilibrium dissociation constant of 2.31 +/- 0.47 nM while in 9-16-day-old cord these parameters were 57.0 +/- 9.4 fmol/mg protein and 2.28 +/- 0.22 nM, respectively. The total binding capacity per cord was 95.8 +/- 8.3 and 121.8 +/- 7.7 fmol/cord for adult and immature rat, respectively. Competition studies using receptor-selective opioid ligands showed that these sites were kappa opioid in nature. Autoradiographical techniques demonstrated a uniform distribution of these sites over transverse sections of 9-16-day-old rat cord. In vitro electrophysiology was performed on spinal cord slice preparations from the 9-16-day-old rat. U69593 (100 nM-1 microM) had no effect on passive membrane properties but produced a naloxone-reversible depression of both spontaneous and electrically evoked activity in dorsal horn neurones. Direct intrathecal injection of U69593 (0.3-10.0 micrograms/animal) into 9-16-day-old rats produced a dose-dependent, naloxone-reversible, antinociception when measured using the paw-pressure test. In conclusion, we have shown that, in contrast to the adult, the spinal cord of the 9-16-day-old rat has a significantly higher concentration of [3H]U69593 binding sites which have functional in vitro and in vivo correlates.