Ortho- and antiorthostatic tests (OT, AOT) were performed in critically ill patients to assess cardiovascular system function. Two types of reactions were observed depending on changes in the cardiac index occurring in response to maximum load (transition from the supine -15 degrees into the upright position +15 degrees). In group 1 (58 tests) transition into the upright position caused a decrease in the parameters of the cardiac pump function, while in group 2 (43 (tests) an increase was observed. The results obtained indicate that these tests increase considerably the diagnostic value of central hemodynamic indexes in the assessment of the circulatory function and functional myocardial reserve in critically ill patients.