Background: Stimulation of arginine vasopressin results in an immediate redistribution of water channels (aquaporin 2; AQP2) in the apical membrane of the collecting ducts, leading to water reabsorption. Water restriction for ≧24 h increases AQP2 proteins in the whole collecting duct which is highest in the inner medulla of the kidney, indicating that dehydration enhances synthesis of this protein. Although increased expression of AQP2 mRNA under this condition has been reported, the increased ratio of mRNA expression in the three regions of the kidney, cortex, outer medulla, and inner medulla, during the dehydration is still unclear. Methods: We investigated the AQP2 transcripts using male Sprague-Dawley rats deprived of water for 24 h. Mimic cDNA for competitive polymerase chain reaction (PCR) was constructed by deleting 180 bp at the middle of a 780-bp partial PCR product for rat AQP2 cDNA. In situ hybridization of the kidney and Northern blotting of inner medulla were performed using a 60-bp oligo-cDNA probe which identified rat AQP2 transcripts in the collecting duct. Results: Dehydration resulted in a significant increase in plasma osmolality and arginine vasopressin concentration and urinary osmolality. Competitive PCR demonstrated that dehydration increased AQP2 transcripts in all parts of the kidney, but was highest in the inner medulla. Northern blotting confirmed the high increased rate of AQP2 transcription in the inner medulla. In situ hybridization showed markedly intensified signals in the inner medulla of dehydrated rats. Conclusions: Our data indicate that dehydration increases the abundance of AQP2 transcripts which may be closely associated with enhancement in AQP2 protein synthesis reported previously. This topographically variable increase in transcription is considered to be one of the mechanisms involved in long-term regulation of water permeability in the collecting duct.