The localization and pharmacological characteristics of vasopressin (VP) binding sites of the V<sub>1a</sub> subtype in developing and adult rat kidney were investigated by radioautography on kidney sections incubated in the presence of a radioiodinated selective V<sub>1a</sub> antagonist. Their localization after in vivo systemic infusion of the radioligand was also investigated. V<sub>1a</sub> binding sites first appear at embryonic day 16 on vascular elements. In the adult, they were localized in the cortex (vascular and tubular structures, juxtaglomerular apparatus), the outer medulla outer stripe (vasa recta) and inner stripe (thin descending limbs of short looped nephrons) and the inner medulla (collecting ducts). Data obtained in vitro were confirmed by in vivo binding at postnatal day 30 (PN30). Whatever their localizations, the V<sub>1a</sub> binding sites exhibited full V<sub>1a</sub> pharmacological profile in postnatal stages rats and in adult rats: a high affinity (n M range) for VP and for the V<sub>1a</sub> agonist, a lower affinity (µ M range) for oxytocin and no affinity for the oxytocin agonist. The presence of V<sub>1a</sub> binding sites in these different structures raises the question of the putative roles of VP in modulating renal functions. A striking finding is the presence of V<sub>1a</sub> binding sites in the outer medullary thin descending limbs of short looped nephrons suggesting their colocalization with urea transporters.