We prospectively evaluated the diuretic effect of furosemide administered by bolus injection and by continuous infusion in 18 cardiac surgery patients. Nine patients were randomly assigned to receive 0.3 mg/kg of furosemide as a bolus injection at time 0 and again 6 hours later (nine patients) or 0.05 mg/kg per hour of furosemide as a constant infusion for 12 hours (nine patients). There were no significant differences between groups with respect to age, weight, creatinine clearance, changes in serum sodium and potassium levels, total urinary concentrations of sodium and potassium, or total urine volume for 12 hours. Diuresis during continuous infusion of furosemide was less variable from hour to hour than after bolus injection of furosemide and was sustained throughout the infusion period. Although the continuous infusion of furosemide will not provide the rapid and vigorous diuresis that is necessary in some clinical situations, it may be useful whenever a gentle, sustained diuresis is desired.