The synthesis of arginine vasopressin (AVP) in the magnocellular neurons of the supraoptic nucleus (SON) and paraventricular nucleus (PVN) is physiologically regulated by both plasma osmolality and volume. To clarify how the regulation of AVP gene transcription is affected by chronic dehydration, we examined changes in transcriptional activities of AVP gene by plasma osmolality and volume in both euhydrated and dehydrated conditions. Euhydrated rats had free access to water while dehydrated rats had been deprived of water for three days before experiments. Rats in both conditions were subject to either acute hypertonic stimuli or hypovolemia, and changes in AVP heteronuclear (hn) RNA levels, an indicator of gene transcription, in the SON and PVN were examined with in situ hybridization. The intraperitoneal (ip) injection (2% body wt) of hypertonic (1.5 M) saline increased plasma Na levels about 40 mEq/l in both euhydrated and dehydrated conditions. However, expression levels of AVP hnRNA in the SON and PVN were increased in only euhydrated, but not dehydrated, rats. On the other hand, ip injection of polyethylene glycol decreased the plasma volume about 16-20%, and AVP hnRNA levels in the SON and PVN were significantly increased in both conditions. Thus, it is demonstrated that signaling pathways regulating AVP gene transcription in the magnocellular neurons were completely refractory to acute osmotic stimuli under the chronic dehydration, and that AVP gene transcription could respond to acute hypovolemia probably through different intracellular signal transduction pathways from those for osmoregulation.