Histochemical activity of malate dehydrogenase (MDH: E.C. 1.1.1.37) in rat sperm mitochondria decreased during the epididymal transit of sperm. In an electrophoretic study two major MDH isozymes (MDH-A and MDH-B) were demonstrated in the sperm. The epididymal sperm showed two minor isozymes associated with MDH-A, while the testicular sperm did not.