The purpose of this study was to determine whether reduction in apparent muscle O-2 diffusing capacity (Dm(O2)) calculated during reduced blood dow conditions in maximally working muscle is a reflection of alterations in blood flow distribution. Isolated dog gastrocnemius muscle (n = 6) was stimulated for 3 min to achieve peak O-2 uptake (V over dot O-2) at two levels of blood flow (controlled by pump perfusion): control (C) conditions at normal perfusion pressure (blood flow = 111 +/- 10 ml . 100 g(-1) . min(-1)) and reduced blood flow treatment [ischemia (I); 52 +/- 6 ml . 100 g(-1) . min(-1)]. In addition, maximal vasodilation was achieved by adenosine (A) infusion (10(-2) M) at both levels of blood flow so that each muscle was subjected randomly to a total of four conditions (C, CA, I, and IA; each separated by 45 min of rest). Muscle blood flow distribution was measured with 15-mu m-diameter colored microspheres. A numerical integration technique was used to calculate Dm(O2) for each treatment with use of a model that calculates O-2 loss along a capillary on the basis of Fick's law of diffusion. Peak V over dot O-2, was reduced significantly (P < 0.01) with ischemia and was unchanged by adenosine infusion at either flow rate (10.6 +/- 0.9, 9.7 +/- 1.0, 6.7 +/- 0.2, and 5.9 +/- 0.8 ml . 100 g(-1) . min(-1) for C, CA, I, and IA, respectively). Dm(O2) was significantly lower by 30-35% (P ( 0.01) when flow was reduced (except for CA vs. I; 0.23 +/- 0.03, 0.20 +/- 0.02, 0.16 +/- 0.01, and 0.13 +/- 0.01 ml . 100 g(-1) . min(-1) . Torr(-1) for C, CA, I, and IA, respectively). As expressed by the coefficient of variation (0.45 +/- 0.04, 0.47 +/- 0.04, 0.55 +/- 0.03, and 0.53 +/- 0.04 for C, CA, I, and IA, respectively), blood flow heterogeneity per se was not significantly different among the four conditions when examined by analysis of variance. However, there was a strong negative correlation (r = 0.89, P < 0.05) between Dm(O2) and blood flow heterogeneity among the four conditions, suggesting that blood flow redistribution (likely a result of a decrease in the number of perfused capillaries) becomes an increasingly important factor in the determination of Dm(O2) as blood flow is diminished.