To clarify the valine catabolism, the activities of principal enzymes in its catabolic pathway, branched-chain aminotransferase, branched-chain 2-oxo acid dehydrogenase complex, methacrylyl-CoA hydratase and 3-hydroxyisobutyryl-CoA hydrolase, were measured using canine tissues. After killing of beagle dogs, tissues (liver, pancreas, kidney, heart, skeletal muscle and mucosae of digestive organs such as stomach, small intestine and colon) were removed and immediately frozen. Branched-chain aminotransferase activity in liver was the lowest among the tissues measured. In contrast, the activities of branched-chain 2-oxo acid dehydrogenase complex in liver as well as in kidney were relatively high and the enzyme complex activities were markedly low in small intestine and skeletal muscle. The activities of methacrylyl-CoA hydratase and 3-hydroxyisobutyryl-CoA hydrolase were relatively high in all tissues, suggesting that a cytotoxic intermediate, methacrylyl-CoA, is immediately degraded to non-toxic compounds, 3-hydroxyisobutyrate and free CoA. These findings suggest that the consumption of branched-chain amino acids in the absorption site (small intestine) is suppressed in order to supply them to the whole body, in particular to skeletal muscle and that skeletal muscle might act as a storage of gluconeogenic amino acids. The high capacity to dispose methacrylyl-CoA produced in the valine catabolism is suggested to play an important role in protecting cells against the toxic effects of methacrylyl-CoA.