The effects of acute volume expansion and hemorrhage on renal nerve activity and renal vascular resistance were examined in chronically instrumented conscious dogs. In six conscious dogs, when the blood volume was expanded by 18 ml/kg, the mean arterial pressure increased by 14 +/- 3 mmHg, mean left atrial pressure increased by 5.3 +/- 0.7 mmHg, and renal nerve activity decreased by 87 +/- 3%, while the renal blood flow was not altered significantly and renal vasoconstriction occurred, i.e., the calculated renal vascular resistance increased by 12 +/- 4% from 0.49 +/- 0.05 mmHg/ml/min. Volume depletion, induced by 20 ml/kg hemorrhage, did not alter the mean arterial pressure (-4 +/- 6 mmHg), while it decreased the mean left atrial pressure by 4.0 +/- 0.7 mmHg and increased the renal nerve activity by 200 +/- 67%. However, the renal blood flow was well maintained at the pre-hemorrhagic control level and renal vasoconstriction did not occur. Thus, in conscious dogs, acute volume changes altered the renal nerve activity dramatically, but these changes in renal nerve activity did not exert any significant effects on renal vascular resistance.