In brief: Holter monitors were used to make ECG recordings on 48 men for six-hour periods on successive days—one day with and one without spontaneous exercise. The subjects were divided into four groups based on frequency of PVCs. There were no significant differences between the four groups' ages or heart rates during the control, exercise, and recovery periods. In subjects with PVCs there was a significant increase in the incidence of PVCs per hour and PVCs per 1,000 heart beats during exercise compared with the control period but no significant difference between exercise periods and the first hour of recovery. More complex arrhythmias occurred during exercise than during the control or recovery periods.