Twenty-three patients with frequent arrhythmias during and shortly after maximal treadmill exercise testing were treated with a single oral dose of a solution of procainamide (15 mg/kg body weight), and the exercise test was repeated 60 minutes later. Plasma concentrations of procainamide were determined at 0, 30, 60, 90, and 120 minutes after the oral dose. If the exertional arrhythmias were suppressed, the patient was retested at another time with 7.5 mg/kg body weight of procainamide; if not, the dose was increased to 22.5 mg/kg body weight. Approximately one half the patients treated with the 15 mg/kg body weight dose showed marked reduction of arrhythmias on the second test. The other half showed incomplete suppression when testing was repeated using a higher dose. A plasma level of 4 to 8 µg/ml procainamide was usually effective, and the frequency of arrhythmias increased as plasma concentrations fell. Maximal oxygen uptake was not changed, but systolic blood pressures, at rest and with exercise, were lower, and ST-segment depression, by computer, was less.