An intravenous infusion of dopamine at 2.5 microgram/kg/min was administered for 40 minutes to anesthetized cardiac surgical patients, and their renal function was measured. Five patients had the usual preoperative regimen of reduced fluid intake for the night and morning before surgery (nonhydrated), and five patients received normal saline, 2 mL/kg/hr intravenously, for 6 hours before anesthesia (hydrated). Renal function (measured by urine output, sodium excretion, free water clearance, and fractional excretion of sodium) was similar immediately before starting the dopamine infusion. All four variables were significantly higher in the hydrated group after 10 minutes; this difference becoming maximal after 40 minutes. Twenty minutes after stopping the dopamine infusion, renal function was similar in the two groups. This study indicates that preoperatively fluid-restricted patients demonstrate powerful salt and water conservation with reduced natriuretic and diuretic responses to a low-dose dopamine infusion when compared with hydrated patients. Patients with adequate fluid loading and intravascular volume will demonstrate a marked natriuresis and diuresis in response to low-dose dopamine infusion.