Water restricted (WR) rats exhibit a rapid suppression of plasma corticosterone following drinking. The present study monitored Fos-like immunoreactivity (Fos) to assess the effect of WR-induced drinking on the activity of vasopressin (VP)-positive magnocellular and pavocellular neurons and corticotropin releasing hormone (CRH)-positive parvocellular neurons in the paraventricular nucleus (PVN) of the hypothalamus. Adult male rats received water for 30 min (WR) in the PM each day for 6 days and were sacrificed without receiving water or at 1 hr after receiving water for 15 min. In WR rats, Fos increased in VP magnocellular and parvocellular neurons, but not CRH neurons. After drinking, Fos was reduced in VP magnocellular and parvocellular neurons, but did not change in CRH neurons. To assess the severity of osmotic stress, rats were sampled throughout the final day of WR. Plasma osmolality, hematocrit and plasma VP were increased throughout the day prior to PM rehydration, and plasma ACTH and corticosterone were elevated at 1230h and 1430h, respectively, showing that WR activates HPA activity during the early PM prior to the time of rehydration. To determine the effects of WR-induced drinking on CRH neurons activated by acute stress, WR rats underwent restraint. Restraint increased plasma ACTH and corticosterone and Fos in CRH neurons; although rehydration reduced plasma ACTH and Fos expression in VP neurons, Fos in CRH neurons was not affected. These results suggest that inhibition of VP magnocellular and parvocellular neurons, but not CRH parvocellular neurons, contribute to the suppression of corticosterone after WR-induced drinking.