Uptake of glucose, free fatty acids (FFA) and lactic acid was studied in soleus muscle of heparinized cats under pentobarbital anesthesia. Cats were separated in 3 groups: caged, "free" and exercised. Blood samples were taken from an artery and the venous effluent of the muscle, a) at rest, b) after contraction, c) after Intralipid infusion, d) after a post-infusion contraction. Glucose was released in some muscles in a) and b), but uptake predominated in c) and d). There was a direct relationship between glucose uptake and lactic acid production only in muscles of exercised cats in a). Venous FFA levels correlated with lactic acid release in b) and d). Results indicated that in the studied conditions soleus muscle of the cat, which is formed by slow oxidative fibres, utilized FFA as the main fuel; glycogen was used during contraction producing a non significant increase in lactic acid. The inhibition by FFA of pyruvate entrance to the TCA cycle, was suggested by the direct correlation between venous FFA levels and lactic acid release.