The ontogenic expression of mRNAs encoding the V1a and V2 vasopressin receptors (V1aR and V2R) was visualized in liver and kidney of embryonic, developing, and adult rats using in situ hybridization histochemistry. Transcripts were detected at 16 and 19 days gestational age in kidney, with V1aR mRNA predominating in the developing cortex and V2R in the medulla. V1aR transcripts in 1-day-old kidneys were in vascular elements, in cells of developing medullary collecting ducts, and over mesangial cells of deep glomeruli, consistent with a role for the V1aR in regulating cellular growth. Expression of V1aR mRNA in the adult was found mainly in medullary vascular elements, arcuate and interlobular arteries, short segments of the cortical distal tubule, and transitional epithelium and smooth muscle of the pelvic wall and ureter. V2R mRNA, at 16 and 19 days gestational age, was in cells of developing medullary and cortical collecting ducts and, after birth, in cells of differentiating thick limbs of the loops of Henle, papillary surface epithelium, overlying macula densa, and short distal nephron segments. This distribution is in accord with the known role of V2 receptors in regulating water excretion. In contrast to kidney, liver did not express V2R mRNA and expressed V1aR transcripts only after birth. V1aR mRNA labeling was over cells bordering central veins on day 1 and surrounding central veins by day 5. A gradient was maximal on postnatal day 21, with V1aR mRNA most abundant in hepatocytes surrounding central veins and virtually absent in periportal hepatocytes. By day 60, most hepatocytes expressed V1aR transcripts, and the gradient was reduced. The ontogenic expression and receptor mRNA gradient are consistent with a role for hepatic V1a receptors in glucoregulation. These experiments confirm the presence of both V1a and V2 receptors in kidney and show that vasopressin receptor mRNA expression is developmentally regulated and tissue specific.