The effects of lead exposure have been studied upon the behavioural and diuretic responses to the kappa-opioid receptor agonist U-50488H in neonatal rats. Lead was administered in the maternal drinking water (100, 300 and 1000 ppm) from conception to postnatal day 14. The hyperactivity, wall climbing behaviours and diuretic effects of U-50488H (0.1-30 mg/kg, i.p.) in 5- and 20-day-old rat pups were unaffected at all 3 lead dose levels. Lead treatment per se produced a decrease in activity at 20 days. These results contrast with our previously reported disruption of mu- and delta-opioid receptor systems following perinatal lead exposure and suggest that the toxic effects of this metal may be confined to particular types of opioid receptor.