Ambulatory electrocardiographic recordings were obtained from 35 male athletes between 14 and 16 years old, and from 35 male non-athlete controls of the same ages, in order to determine the effects of regular physical training on cardiac electrical activity. In the young athletes, the heart rates were significantly (P less than 0.01) lower than in the nonathletes. Sinus intervals over 2.00 s were present in five athletes (14%) and one control (3%). First-degree atrioventricular block was detected in eight athletes (23%) and four controls (11%), and second-degree block was detected in seven athletes (20%) and one control (3%) (P less than 0.05). Ventricular premature beats were present in 60% of athletes and 57% of controls. The bradycardia in athletes did not predispose to ventricular ectopic activity, since heart rates at the times of occurrence of extrasystoles were higher in athletes than in controls. Even after two years of regular physical training there are significant differences in sinus nodal function and atrioventricular conduction as between young athletes and controls.