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The activities of malate dehydrogenase (MDH) and lactate dehydrogenase (LDH) and the pattern of the isoenzymes of LDH were determined in the peripheral blood leukocytes of dogs, rabbits and cats. Rabbits had significantly higher plasma glucose concentrations than dogs or cats. Feline leukocytes showed higher LDH and lower MDH activities than canine or rabbit leukocytes. The M/L ratio, defined as the MDH activity divided by the LDH activity in cytosolic fractions, was considered to be a good indicator with which to evaluate the metabolic state in animal tissues. The M/L ratio was highest in canine and lowest in feline leukocytes. LDH-2 and LDH-3 isoenzymes were dominant in canine leukocytes. LDH-1 and LDH-2 were dominant in rabbit leukocytes, whereas LDH-5 was dominant in feline leukocytes. It was evident that there were significant differences in energy metabolism between the leukocytes of dogs, rabbits and cats.