Using 24-hour Holter ECG examinations, the authors assessed ECG changes in a group of 86 healthy students of a Bratislava secondary school. The students were 14-18 years old. Their mean age was 15.8 years. As to ECG changes, the authors evaluated changes of the heart rate, ectopic disorders of the cardiac rhythm and right bundle branch block. Almost in all students the authors detected one or several disorders of the heart rhythm. Extreme changes were recorded in particular in the heart rate, i. e. tachycardia and bradycardia. Sinus arrhythmia was present in 80.2% of the adolescents, right bundle branch block in 57%, and as to ectopic disorders, ventricular extrasystoles were present in 51.2% of the students and supraventricular ones in 34.9%.