The authors studied 513 males, permanently living in the high-mountain regions of Tian Shan and the Pamirs (2800 - 4000 m above sea level). A control group consisted of 404 males permanently living at low altitudes (780-900 m above sea level) in the Kemin District, Kirghiz SSR. The probands' ages were 30-59 years. In all of them the resting electrocardiograms were recorded; 110 exercise tests were made in the high mountains, and 35 tests, at the low altitudes. The prevalence of heart rhythm disturbances was statistically significantly higher in the inhabitants of the high-mountain regions (12.1%) than in the low-altitude inhabitants (2.9%; p less than 0.0001). The most frequent disturbance was the 1st-degree a-v block (6 per cent). In the high mountains cardiac arrhythmias are usually associated with right ventricular hypertrophy, caused by high-altitude hypoxia. During exercise heart arrhythmias appeared conspicuously less frequently in the high mountain than in the low altitude inhabitants.