The "Minsk-22" computer was used to study the rhythm of cardiac contractions. The R - R intervals of ECG tracings were studied in 1000 to 2000 cardiac cycles. The measurement of these intervals was performed automatically by means of a special device - intervalmeter. The data input was done from a punch tape. Mathematical analysis of the cardiac rhythm consisted in calculating the mathematical expectance, excess and asymmetry of the distribution of the R - R intervals of ECG. This method was employed for the analysis of the sinus rhythm in normal individuals and in patients with atherosclerosis and rheumatic heart diseases. The rhythm was recorded at rest and during functional tests (administration of nitroglycerine, or single breath holding at the peak of inspiration). The asymmetry of the R - R intervals distribution is an index that helps to reveal the dynamics of the transitory processes, being in this respect superior to the intervalgramme. The excess index permits to evaluate quantitatively the stability of the rhythm and the peculiarities of its regulation. Employment of the described methods of cardiac rhythm analysis is expedient for various functional tests and the assessment of the pharmacological effects.