After 24-h water deprivation, five men (23-41 yr; 78 +/- 3.6 kg) consumed, within 4.0-6.2 min, 12 mL/kg of one of six fluid formulations (16.5 C) once a week over a period of 6 weeks: water, hypotonic saline (0.045% Na+), isotonic saline (0.36% Na+), hypertonic glucose (9.7% glucose), and two commercial mildly hypertonic 9.7% carbohydrate drinks. Blood samples were drawn 5 min before and 3, 9, 15, 30, and 70 min after completion of drinking. Ingestion induced no significant change in plasma Na+, K+, osmotic, or protein concentrations; blood pressure; or heart rate. Plasma volume (PV) was increased (P < 0.05) between 30-70 min with isotonic saline and the two commercial drinks. Ingestion induced a decrease in plasma AVP (PAVP) at 3 min, which was maximal (P < 0.05) at 15 min with all drinks. Thus, the act of drinking, independent of the composition or osmolality of the fluid absorbed, leads to a prompt inhibition of PAVP secretion in man. With the exception of rehydration with isotonic saline, this prompt response was followed by a long lasting inhibition of PAVP. There was no change in PRA, plasma aldosterone, atrial natriuretic peptide, or epinephrine, but an increase in plasma norepinephrine occurred immediately after ingestion, which suggests, like that for PAVP depression, a drinking-stimulated neural mechanism.