The rate of DOPA accumulation after the administration of a decarboxylase inhibitor (an in vivo estimate of dopamine synthesis) was determined in rat posterior pituitary, median eminence and striatum, regions containing terminals of tuberohypophyseal, tuberoinfundibular and nigrostriatal dopamine neurons, respectively. 3 days of water deprivation increased the hematocrit and the plasma sodium concentration and also increased the rate of DOPA accumulation in the posterior pituitary, but not in the striatum or median eminence. Water deprivation or substitution of 2% NaCl for drinking water for 5 days increased DOPA accumulation in the posterior pituitary and the plasma sodium concentration, while the hematocrit was increased only in the water-deprived group. Following 3 days of water deprivation, access to water for 3 h caused the elevated DOPA accumulation in the posterior pituitary and the plasma sodium concentration to return to control values while the hematocrit remained slightly elevated. Substitution of 2% NaCl for drinking water for 48 h to rats which had been water-deprived for 3 days restored the hematocrit to control, but did not alter the water deprivation-induced increase in plasma sodium concentration or the rate of DOPA accumulation in the posterior pituitary. These results suggest that tuberohypophyseal dopaminergic neurons are regulated, at least in part, by sodium or osmoreceptors.