Succinic dehydrogenase activity and lactic dehydrogenase activity has been measured in myocardial homogenates from guinea pigs, rabbits and dogs born and bred at an altitude of 4300 m and compared with observations made on similar specimens from sea level animals Substantial differences in the activities of both these enzymes were observed between the three species. Within each species there was a significant increase in succinic dehydrogenase activity in the high altitude animals. Some degree of increase in lactic dehydrogenase activity was observed in high altitude guinea pigs but the differences between high altitude and low altitude rabbits and dogs were not statistically significant. © 1970.