Evidence from behavioral studies suggests that the process of weaning activates the development of a delta-opioid receptor subtype. We now report the influence of weaning on the development of delta receptors in the central nervous system assessed by membrane homogenate binding and autoradiography with selective delta radioligands and by in situ hybridization using a cRNA probe for the delta receptor. Binding was carried out by using [3H][D-Ala2]deltorphin I (DELT I), [3H]IIe5,6-deltorphin II (IIe5,6-DELT II) and [3H]naltrindole (NTI). [3H]IIe5,6-DELT II and [3H]NTI labeled an equivalent number of sites in brain and spinal cord from both weaned and nonweaned 25-day-old rats. The number of sites labeled by [3H]DELT I was similar in nonweaned rats but significantly higher in the brain and cord from weaned animals. Furthermore, the ontogenetic profile of these three ligands was distinct. Quantitative autoradiography showed identical levels of [3H]IIe5,6-DELT II binding in all brain regions in weaned and nonweaned rats. In contrast, levels of [3H]DELT I binding were significantly higher in weaned rats and this difference was localized to the deep layers of the frontal-parietal cortex and to the pontine nucleus. In situ hybridization experiments showed no differences in delta-opioid receptor mRNA density between weaned and nonweaned groups in the regions in which binding differences were observed. Weaning stimulates the development of a subpopulation of delta receptors recognized by [3H]DELT I but not by [3H]IIe5,6-DELT II or NTI. This effect is localized to specific brain regions and does not appear to reflect increased synthesis of mRNA coding for the delta receptor.