R ecent studies have revealed that arginine vasopressin (AVP) has at least two types of receptors in the kidney: V1a receptor and V2 receptor. In this study, microlocalization of mRNA coding for V1a and V2 receptors was carried out in the rat kidney using a reverse transcription and polymerase chain reaction. Large signals for V1a receptor PCR product were detected in the glomerulus, initial cortical collecting duct, cortical collecting duct, outer medullary collecting duct, inner medullary collecting duct, and arcuate artery. Small but detectable signals were found in proximal convoluted and straight tubules, inner medullary thin limbs, and medullary thick ascending limbs. Large signals for V2 receptor mRNA were detected in the cortical collecting duct, outer medullary collecting duct, and inner medullary collecting duct. Small signals for V2 receptor were found in the inner medullary thick limbs, medullary thick ascending limbs, and initial cortical collecting duct. Next, we investigated V1a and V2 receptor mRNA regulation in the dehydrated state. During a 72-h water restriction state, the plasma AVP level increased and V2 receptor mRNA decreased in collecting ducts. In contrast, V1a receptor mRNA did not change significantly. Thus, the two AVP receptor subtypes are distributed differently along the nephron, and these mRNAs are regulated differently in the dehydrated state.