Three groups of Sprague-Dawley rats were kept at the ambient temperatures of 8, 24 and 32 degrees C respectively for 3 to 7 days. These animals maintained their rectal temperatures within normal limits. However, the animals of either the heat-stressed or the cold-stressed group yielded not only the functional but also the organic effects to their hearts when compared to those of the control group (24 degrees C exposure). Heat exposure produced a lower heart rate and higher cutaneous temperature, while cold exposure produced a higher heart rate and a lower cutaneous temperature. In addition, the ECG recordings showed a predominant S wave and probably a depressed S-T segment in response to either heat or cold exposure. At the end of either 3-day or the 7-day thermal exposure, the animals were subjected to decapitation. Their hearts were sampled for both light and electron microscopic examinations. It was found that the heat-stressed animals shared with the cold-stressed animals a common picture of the myocardial changes. The subcellular changes in the myocardium in response to external heat or cold were characterized by the mitochondrial hypertrophy, the intracellular edema, the destruction of myofibrils, the dilatation of the intercalated discs, and some abnormalities in their capillaries. The data demonstrate that a short term (3 to 7 days) of moderate heat (32 degrees C) or cold (8 degrees C) exposure produces myocardial lesions in rats, although they maintain their body temperatures within normal limits.