Among 126 top Israeli athletes, in whom an ECG was obtained during a random survey, 11 had first-degree heart block (P-R greater than or equal 0.21 sec.) and in three of them Wenckebach's phenomenon was found. The latter could be demonstrated only after 15 minutes' rest in a recumbent position and was abolished by sitting, standing, and the administration of atropine. This subjects with Wenckebach's phenomenon were followed for 6 years. The heart block was found to be present only during seasons of intensive training and could not be demonstrated a few weeks after the training was reduced in intensity or stopped. No heart disease or diminution of performance developed during 6 years of follow-up. Transient second-degree heart block in top athletes is probably much more frequent than hitherto suspected, but it can be demonstrated only if the athlete is examined during rest and in the recumbent position. It is assumed to be a physiological phenomenon related to heavy physical training.