Two patients with chronic renal failure developed transient sinus node dysfunction requiring insertion of a temporary pacemaker while receiving procainamide to control ventricular arrhythmias. Blood levels of procainamide were found to be elevated, although at these levels, sinus node dysfunction has not previously been reported. Following discontinuance of procainamide, sinus rhythm returned. A combination of factors, including elevated levels of N-acetyl procainamide, the metabolite of procainamide with anti-arrhythmic properties, are suggested as possible contributory causes for the ECG findings. Thus, procainamide may produce electrophysiologic features of "sick sinus syndrome" in patients with chronic renal failure even when blood levels of this substance are being monitored.