A large number of papers in the literature have reported peak blood flows in electrically stimulated isolated skeletal muscles in the range of 40-140 ml.min-1.100 g-1. Peak muscle blood flows of similar magnitude have been measured in human subjects with 133Xe clearance and plethysmography. In contrast, measurements of average blood flow in conscious animals at VO2max with radiolabeled microspheres have ranged from about 150-250 ml.min-1.100 g-1. Furthermore, flows in active high oxidative muscles in conscious animals during maximal exercise may attain 500 ml.min-1.100 g-1. Reasons for these apparent discrepancies are unknown, but possible explanations including technical artifacts and differences in mechanical influences on contracting muscles in isolated preparations and exercising conscious animals are discussed. It is concluded that the primary reason for the differences in magnitude of muscle blood flow is the efficient muscle pumping action that occurs in vivo during locomotory exercise.