Ambulatory monitoring of the electrocardiogram was performed in 131 healthy boys aged between 10 and 13 years for two consecutive periods of 24 hours. When awake the maximal heart rates ranged from 100 to 200 and the minimal from 45 to 80 beats per minute. During sleep maximal rates were 60 to 110 beats and minimal rates 30 to 70 beats per minute. Sinus arrhythmia was seen in every boy and in 36 (27.5%) no other changes were found. Sinuatrial block, Mobitz type I, was not seen. Sinuatrial block, Mobitz type II, occurred twice only. Complete sinuatrial block occurred in 8.4 per cent, never lasted more than one cycle, and was always followed by a junctional beat. First degree atrioventricular block occurred in 8.4 per cent and Mobitz type I atrioventricular block in 10.7 per cent. Premature beats were always single, atrial in 13 per cent, ventricular in 26 per cent, and except in two boys were never more than four in 24 hours. There were no episodes of ventricular or supraventricular tachycardia. Changes in P wave morphology were common and slow junctional rhythm occurred in 13 per cent during sleep.