The ontogenesis of kappa-opioid receptors has been studied in the postnatal period from day 5 to day 30 using the highly selective kappa-site ligand [3H]U-69593 in binding studies. Analyses of saturation curves revealed a marked increase in the binding capacities between day 5 and day 10 with no further increment in the number of sites up to adult ages confirming a distinct ontogenetic profile from mu- and delta-sites. When expressed per mg protein the number of sites declined from day 10 to adult. At all postnatal ages there was little change in receptor affinity. This ontogenetic profile is broadly in agreement with studies using non-selective kappa-ligands but the number of sites labelled by [3H]U-69593 is markedly lower in both the neonate and the adult.