The present investigation on human skeletal muscle demonstrated that: 1. There was a correlation between the total lactate dehydrogenase activity and fibre type distribution. The LDH activity decreased with increasing percent slow twitch (ST) or type I fibres. 2. The decrease in total LDH activity was related to an elevated relative activity of the heart specific LDH (H-LDH) isozymes in the ST fibres. 3. The LDH isozyme pattern was influenced by environmental factors such as training. The relative activity of H-LDH increased after endurance training. However, extreme anaerobic training did not induce any measurable changes neither in total LDH activity nor in LDH ioszyme pattern. 4. Strong product inhibition of total LDH activity was present even at physiologically low product concentrations in an optimal in vitro system indicating a regulatory function of LDH in the glycogenolysis in vivo during physical activity. 5. The intracellular distribution of the LDH isozymes in human skeletal muscle was in good agreement with earlier findings in animal tissues. LDH of predominant muscle specific type was found to be bound to the sarcoplasmic reticulum and the outer mitochondrial membrane, while LDH of predominant heart specific type was found in the mitochondria.