['Effective material exchange between blood and tissue depends on the heterogeneity of microvascular flow. The objective was to address inconsistencies between intravital studies regarding this dependency. We tested the hypothesis that heterogeneity of red blood cell velocity (VRBC) in capillary beds varies with the strength of metabolic stimulus and with capillary bed geometry.', 'We used videomicroscopy to measure VRBC in a bed of 10-24 capillaries at the surface of extensor digitorum longus (EDL) muscle in anesthetized rats. The coefficient of variation (CV = standard deviation/mean; an index of spatial heterogeneity) was computed in the same bed before and after (i) 1, 2, 4, or 8 Hz supramaximal muscle contraction or (ii) adenosine superfusion (10(-7)-10(-3) M). Beds with or without arteriolar-venular capillary shunts were used.', 'Although control VRBC differed between beds (shunt: 232 microns/s; no shunt: 130 microns/s), the percentage increases in postcontraction VRBC did not (range: 111-326%). In both beds, control CV varied greatly (overall range: 28-117%) and 2-8 Hz muscle contractions reduced CV significantly by 25%. Similar results were obtained for adenosine. In confirmatory experiments using the rat cremaster muscle, contractions (4 Hz) and adenosine (10(-4) M) also reduced CV. Based on all data, CV = 63-0.022 VRBC (r = 0.82, P < 0.001).'] Does heterogeneity of red blood cell velocity in skeletal muscle decrease with increased flow?