The present experiments were undertaken to study the effects of FFA on lactate production by skeletal muscle during stimulation. In the first group, dogs were anesthetized with sodium pentobarbital and given no anticoagulant. The second group was also anesthetized with sodium pentobarbital but in addition given heparin and a fat-albumin infusion to elevate FFA. Stimulating the nerves to a group of skeletal muscles in the hindlimb (1.5/s) increased muscle blood flow 2.4-fold in both groups. In the first group stimulation did not alter the arteriovenous difference of lactate across the muscles. The difference was close to zero before and during stimulation. However in the second group, in which FFA were elevated, stimulation produced a large increase in muscle lactate production. In both groups there were no differences in the L/P ratio of muscle venous blood during stimulation. These results indicate that an increase in lactate production following muscle stimulation is not necessarily related to a state of tissue hypoxia.