In experiments on rats, changes of the ultrastructure of myocardial histo-hematic barriers in the right atrium during short-term and prolonged heat stress were studied. Short-term heat exposure (40 degrees C, 30 and 60 min) increased the rectal temperature by 3.0-3.5 degrees C. Immediately after heat exposure and 24 and 48 h following the exposure, the destructive and compensatory adaptive alterations in the microcirculatory bed were detected, accompanied by an edema of the pericapillary space and the changes in the size of mitochondria under the cardiomyocyte sarcolemma. Prolonged heating (40 degrees C, 4 h daily for 28 days) led to an increase in the rectal temperature by 0.4-1.6 degrees C. Changes in the microcirculatory bed had both dystrophic and compensatory adaptive nature. Erythrocyte and thrombocyte sludge and erythrocyte lysis were found in the lumen of microvessels. The number of endocytotic vesicles in endotheliocytes was variable; endotheliocyte nuclei had increased electron density and irregular outline. Myelin-like structures and apoptotic bodies were found in the pericapillary spaces. Under the cardiomyocyte sarcolemma the accumulation of mitochondria and sarcoplasmic vacuolization were noted. Comparison of short- and long-term effects of heat on histohematic barriers in the right atrium shows more significant changes after prolonged heat stress.