In 128 mature white rats (males) after adaptation for 1--45 days, to 4,000 m altitude (North-West Pamir, Glasier Fortambek), weight characteristics of the heart were estimated (capillary diameters, their network density per 1 mm2 of the section summing length and volume in 1 mm3 of the right and left ventricular myocardium. Since the 21st day of the animals' staying at the altitute mentioned, a stable increase in the ventricular index occurs demonstrating a predominant right ventricular hyperfunction; the blood capillary volume in the left ventricular myocardium, during the whole time of the experiment, remains at the control levels, at the expense of the concomitant changes in the diameter and density. The blood capillary volume in the right ventricular myocardium, beginning from the first day of the experiment, increase essentially; this increase was kept up to the 14th day of the experiment, but, nevertheless, it remains at the control levels, at the expense of the concomitant changes in the diameter and density. The blood capillary volume in the right ventricular myocardium, beginning from the first day of the experiment, increase essentially; this increase was kept up to the 14th day of the experiment, but, nevertheless, it remains at the control levels, at the expense of the concomitant changes in the diameter and density. The blood capillary volume in the right ventricular myocardium, beginning from the first day of the experiment, increase essentially; this increase was kept up to the 14th day of the experiment, but, nevertheless, it did not exceed the volume indices of the left ventricular capillaries.