Changes in the activities of enzymes related to energy metabolism in the testicular tissues of dogs with seminoma were investigated. The testis was removed surgically from animals anaesthetized with halothane. Cytosolic and mitochondrial fractions were isolated and the total RNA was extracted from testicular homogenates. The activities of enzymes related to energy metabolism were measured and the mRNA of cytosolic malate dehydrogenase (MDH) was investigated by the reverse transcriptase-polymerase chain reaction (RT-PCR). The activities of the glycolytic enzymes glucose-6-phosphate dehydrogenase (G6PD) for the pentose phosphate pathway and malate dehydrogenase (MDH) for the malate-aspartate shuttle, and the expression of the mRNA of cytosolic MDH were significantly increased in the testicular tissues of dogs with seminoma. These enzymatic activities may be useful indicators with which to evaluate changes in the metabolic conditions in testicular tissues of dogs with seminoma.