In acute experiments performed on rats placed in heat-chamber (40 degrees C, 30- and 60-minutes-long exposure), an increase in NADPH-diaphorase (NADPH-d) and succinate dehydrogenase (SDH) activities were found in the neurons of intramural ganglia of the right atrium, while SDH and lactate dehydrogenase (LDH) activities were increased in cardiomyocytes. Hormone-producing cardiomyocytes, containing atrial natriuretic peptide, had numerous large secretory granules under the sarcolemma close to the microvessels. In contractile cardiomyocytes, the signs of early stage of apoptosis were observed including the subsarcolemmal accumulation of small mitochondria and cytoplasmic invaginations into the nucleus. Prolonged heating (37 degrees C, 4 h daily) led to an increased activities of NADPH-d and SDH in neurons of intramural ganglia of the right atrium and of NADPH-d and LDH in cardiomyocytes. Secretory granules in hormone-producing cardiomyocytes were small and reduced in number. Cardiomyocytes showing the signs of the late stage of apoptosis (with nuclear and cytoplasmic condensation) were demonstrated.