UDC 612.213:612.172.2 Ever increasing attention is being paid nowadays to the study of the characteristics of the sinus rhythm of the heart as an important source of information on a person's state. This information is obtained from long-term continuous recording of the ECG (2-5 min), by calculating various statistical characteristics of the sequence of RR intervals of the ECG. This approach has been used to study neurohumoral regulation of the cardiac rhythm [i] and also to study problems in applied physiology. In particular, investigation of the characteristics of the sinus rhythm is widely used in aviation and space physiology [2], in sport physiology [3], work physiology, and engineering psychology [4]. Finally, analysis of the cardiac rhythm is becoming increasingly important in clinical physiology. It has been found that the greatest change in the sinus rhythm (physiological arrhythmia) occurs in healthy people; when pathological states (essential hypertension, angina pectoris, myocardial infarction, burns, etc.) arise the intensity of the physiological arrhythmia is reduced and an absolutely regular pulse may be established [5]. These features of the behavior of the sinus rhythm have enabled its analysis to be effectively used as a means of monitoring preoperative preparations and the patient's state during operations [6], to predict postoperative complications [7], for individual selection of drugs for the treatment of disturbances of cardiac rhythm [8], and in many other cases. During investigation of the characteristics of the sinus rhythm of the heart, attempts have been made to increase their information content by the use of various function tests, The orthostatic test [9] and graded physical exertion [i0] are most frequently used. Various pharmacological tests, such as injection of atropine [ii], also are used. A promising trend in the investigation of variability of the sinus rhythm is the development of methods for the objective evaluation of the severity of a patient's state on this basis. To increase the informativeness of the investigation, function tests must evidently be useful in this case also. The function tests already mentioned, as a rule, cause an increase in the pulse rate and a decrease in physiological arrhythmia; however, the cardiac rhythm may also change in the same direction under the influence of diseases. Accordingly, in patients with acquired heart disease the results of a function test with physical exertion have proved uninformative [12]. This points to the need for carrying out a function test which would cause a shift in the opposite direction, i.e., which would increase the dispersion of the rhythm. Breathing with increased depth of inspiration has been shown to have this effect [13]. This suggested that the change in variability of the sinus rhythm of the heart during deep breathing might prove a useful indicator of the functional state of the subjects tested. The object of the present investigation was to test this hypothesis, EXPERIMENTAL METHOD Three groups of patients with suppurating wounds, admitted for treatment to the A. V. Vishnevskii Institute of Surgery, Academy of Medical Sciences of the USSR, were investigated, Laboratory of Physiology, A. V. Vishnevskii Institute of Surgery, Academy of Medical Sciences of the USSR, Moscow. (Presented by Academician of the Academy of Medical Sciences of the USSR M. I. Kuzin.) Translated from Byulleten' Eksperimental'noi