SDH-activity in m. gastrocnemius muscle tissue of the Lean and Fat mice. Striated muscle from regio cruris (m. gastrocnemius) was used to determine mitochondrial succinate dehydrogenase (SDH) activity levels. 12 of each Lean and Fat line mice were analysed and 150 muscle fibers per mouse were measured for densitometry. This histochemical assay demonstrated marked (*P < 0.001) increased level of SDH activity in muscle fibers of Lean mice suggesting pronounced increase in oxidative metabolism in this line. % densitometry with standard errors are displayed - lower transparency of muscle histological sections in the Lean mice resulted from higher SDH-enzyme activity which in turn gave lower % densitometry readings in the Lean line.