Exposure of rats to hypobaric hypoxia (9.5% O2) for 40 days produced cardiac hypertrophy, increases in the total LDH activity and subunit M percentage of LDH and a significant decrease (50%) in H/M ratio when compared to control rats. An equally long exposure to cold (+6 degrees C) did not affect heart weight or total LDH activity, but a significant increase (17.6%) in the H/M ratio developed. Simultaneous exposure to cold (+6 degrees C) and hypoxia (9.5% O2) caused a significant decrease in the H/M ratio, but there was no change in the total LDH-activity when compared to the control group. The poor hypoxic tolerance of the cold-exposed rats could partly be explained by disadvantageous distribution of LDH isoenzymes leading to impaired glycolytic capacity of such an aerobic organ as the heart.