The rate of dopamine (DA) synthesis (DOPA accumulation after the administration of an inhibitor of aromatic L-amino acid decarboxylase) was determined in terminals of tuberohypophyseal DA neurons in the neurointermediate lobe (NIL) of the rat hypophysis at various times after the presentation of osmotic stimuli. DA synthesis in the NIL, but not in median eminence or striatum, was increased 24 h but not 4 h after an injection of hypertonic saline (5 ml 15% NaCl/kg, s.c.), provided the animals were not permitted access to water after the injection; 24 h of water deprivation per se was without effect on the rate of DA synthesis. DOPA accumulation in the NIL was also increased after an intravenous infusion of mannitol (25 ml 20% mannitol/kg) plus 24 h of water deprivation. These results suggest that the dehydration-induced activation of tuberohypophyseal DA neurons which is normally seen after 2-3 days of water deprivation can be accelerated if water deprivation is preceeded by injections of hypertonic saline or mannitol.