The expression of arginine vasopressin (AVP) gene in the paraventricular (PVN) and supraoptic nuclei (SON) was investigated in rats with lithium (Li)-induced polyuria, using in situ hybridization histochemistry and radioimmunoassay. The male Wistar rats consuming a diet that contained LiCl (60 mmol/kg) for 4 weeks developed marked polyuria. The Li-treated rats produced a large volume of hypotonic urine with low ionic concentrations. Plasma sodium concentrations were found to be slightly increased in the Li-treated rats compared with those in controls. Plasma concentration of AVP and transcripts of AVP gene in the PVN and SON were significantly increased in the Li-treated rats compared with controls. These results suggest that dehydration and/or the activation of visceral afferent inputs may contribute to the elevation of plasma AVP and the upregulation of AVP gene expression in the PVN and the SON of the Li-induced diabetes insipidus rat.