Effects of acute volume expansion with isotonic isoncotic 3% dextran in saline were examined on renal nerve activity (RNA), renal blood flow, vascular resistance, and sodium and water excretion in conscious dogs. In intact dogs, acute volume expansion increased mean arterial pressure 15 +/- 3 mmHg, left atrial pressure 5.5 +/- 0.6 mmHg, and decreased RNA 88 +/- 2%, whereas renal blood flow did not change and renal vascular resistance increased slightly. When renal perfusion pressure was maintained at control levels, volume expansion decreased RNA 87 +/- 2% and renal vascular resistance 15 +/- 4%. During the 80-min period after volume expansion, urine flow rate increased 0.66 +/- 0.13 ml/min and sodium excretion rose 3.89 +/- 0.54 mueq X min-1 X kg-1, whereas RNA remained depressed. Arterial baroreceptor denervation (ABD) did not diminish responses of RNA, renal blood flow, renal vascular resistance, or sodium and water excretion to volume expansion. After ABD plus bilateral cervical vagotomy, volume expansion did not decrease RNA, and diuretic and natriuretic responses were significantly attenuated (P less than 0.025). However, responses of renal blood flow to volume expansion were not altered significantly. In conscious dogs with renal denervation, responses of renal blood flow to volume expansion were not impaired, whereas diuretic and natriuretic responses were attenuated (P less than 0.025). Thus, in intact conscious dogs, vagally mediated reflex decreases in RNA induced by acute volume expansion exerted a significant effect on sodium and water excretion but little control of renal blood flow and renal vascular resistance.