Leg and splanchnic exchange of free fatty acids (FFA) and glucose was determined during 90 min bicycle exercise after heparin administration in six healthy male volunteers, and the results compared to those obtained previously under identical exercise conditions without the administration of heparin. Heparin elicited a marked elevation of plasma FFA at rest but the exercise values were not higher than during exercise without prior heparin administration. The FFA uptake in the exercising legs was, however, augmented after heparin due to an increase in the fractional leg uptake. Leg glucose uptake and splanchnic glucose production during exercise were not affected by heparin administration. It is concluded that inhibition of glucose uptake by FFA is of minor importance in the regulation of muscle substrate utilization during exercise in man.