The aim of this study was to investigate whether arginine vasopressin (AVP) is natriuretic in sheep at plasma concentrations comparable to those induced by water deprivation. AVP was infused intravenously at 0.1, 0.2, and 0.5 microgram/h for 24-48 h in sheep allowed free access to water. Infusion of AVP at 0.1 microgram/h did not alter renal Na output, whereas infusion of AVP at both 0.2 and 0.5 microgram/h significantly increased daily output of Na in urine. Significant natriuresis did not occur until 3.5 h after the start of AVP infusion at 0.2 microgram/h. Plasma AVP levels induced by these infusions were 9.8 +/- 1.6 (0.1 microgram/h AVP), 21.9 +/- 7.7 (0.2 microgram/h AVP), and 32.5 +/- 9.0 pg/ml (0.5 microgram/h AVP) after 24 h. These concentrations are within the range found in sheep deprived of water for 3 days. Hypophysectomy abolished increases in plasma AVP concentration but not natriuresis in response to water deprivation. This suggests that increased plasma AVP concentration does not play an essential role in the mechanisms subserving dehydration-induced natriuresis.