The development of vasopressin (AVP) receptors in the rat brain, spinal cord and pituitary gland was studied by in vitro light microscopic autoradiography. AVP binding sites were labeled using [3H]AVP in tissue sections from animals aged between embryonic day 12 (E12) and postnatal day 90 (PN90); the binding of [3H]AVP to oxytocin receptors was prevented by adding in the incubation medium a highly selective oxytocin agonist. Specific binding was first detected at E16 in the ventral pontine reticular formation. Many other brain areas were progressively labeled between E18 and PN5. The distribution of binding sites observed at PN5 remained unchanged until the beginning of the third postnatal week. Thereafter binding was markedly reduced or even disappeared in several areas, in particular in the facial nucleus. The adult distribution of AVP binding sites was established at the time of weaning. The properties of transient AVP binding sites in the facial nucleus were studied both by autoradiography and by electrophysiology. Non-radioactive AVP displaced [3H]AVP binding in this nucleus as efficiently as it did in the lateral septum of the adult. Single-unit extracellular recordings showed that AVP can excite facial motoneurones by interacting with receptors which are pharmacologically indistinguishable from V1 (vasopressor) type. Thus, AVP binding sites transiently expressed in the brain of fetal and infant rat probably represent functional neuronal receptors, having the same ligand selectivity and affinity than AVP binding sites present in the adult. This suggests that AVP acts not only as a neuropeptide in the adult brain but may play a significant role during maturation of the central nervous system.