Six patients with complete heart block provided an opportunity to study renal hemodynamic responses to increases in heart rate produced by an artificial pacemaker. Observations were made at rest and following exercise during idioventricular rhythm and slow rates and were repeated after increasing the rate to 70/min by means of an artificial pacemaker. Ability to excrete a water load was impaired at the slow idioventricular rate; this was associated with a reduced glomerular filtration rate. No acute improvement in renal function occurred with pacemaker-induced increase in heart rate except in the presence of congestive heart failure. With long-term pacing (7 to 8 months), the glomerular filtration rate and the ability to excrete solute-free water increased in all patients studied. The acute increase in free water clearance which occurred with increased heart rate in the presence of congestive heart failure suggests that decreased renal perfusion may be responsible for the inability to excrete excess water. The dilutional hyponatremia in congestive heart failure may be explained on this basis. Changes in indices of cardiac function in these patients were similar to those reported by others. No change in resting cardiac output occurred at heart rates between 40 and 70/min in the absence of congestive heart failure. The response to exercise, however, was greater at the higher rates.