Normal and anemic dog gracilis muscles were compared at equal O2 uptake rates (VO2) to locate the principal site of resistance to diffusive O2 transport. Anemia halved the hematocrit and the number of red blood cells per square millimeter of muscle cross section. Flow doubled in anemia, and flow times arterial O2 content, PO2 of effluent blood, and O2 extraction per red blood cell were approximately the same as control. Nevertheless, intracellular PO2 was significantly lower in anemia. At any instant the aggregate red blood cell surface area for O2 release was about half normal. Because the flux (VO2) was the same as control, the driving force for diffusion from red blood cell to myocyte should have doubled. An estimate of the total driving force from red blood cell to mitochondria was greater in anemia. This increase was much less than a factor of 2 because lower intracellular PO2 increases myoglobin-facilitated diffusion, thus decreasing resistance inside the myocyte. The role of myoglobin and the coupling of convective to diffusive transport are discussed. We conclude that the principal resistance to O2 diffusion lies outside the myocyte.