Abstract To determine the incidence of arrhythmias and conduction disturbances in trained athletes and the level of physical training at which they occur, 24-hour ambulatory electrocardiographic recordings were obtained in 80 healthy runners during both exercise and free activity. Subjects were grouped according to the number of miles per week (mpw) they had regularly run during the previous 3 months: Group I—0 to ≤ 5 mpw (≤ 8 km); Group II—⪢5 to ≤ 15 mpw (⪢ 8 to ≤ 24 km); Group III—⪢ 15 to ≤ 30 mpw (⪢ 24 to <- 48 km)s; and Group IV—⪢ 30 mpw (⪢ 48 km). Ectopic ventricular complexes occurred in 41 of 80 subjects (50%) and ectopic supraventricular complexes occurred in 33 (41%). There were 2 episodes of paired ventricular ectopic activity and a 5-beat run of ventricular tachycardia with exercise. The study revealed no significant differences in the occurrence of arrhythmias or conduction disturbances in the different groups, although the 2 episodes of paired ventricular ectopic activity and 5-beat run of ventricular tachycardia are of concern.