Microvascular red cell (RBC) supply in resting skeletal muscle fluctuates with time and varies from capillary to capillary. Our aim was to quantitate this heterogeneity in two species (frog, rat) with very different oxygen requirements. Video recordings were made of RBC flow in capillaries associated with superficial fibers of frog sartorius and rat gracilis muscles, for periods of 2-10 min. RBC supply rate (cell/s) to individual capillaries was computed as the product of RBC 'content' (measured as lineal density, cells/mm, by a video-densitometric method) and RBC velocity (mm/s, measured by a spatial correlation technique). The videotapes were analyzed at rates of 6 to 15 samples per second. Regression analysis of the mean data showed little or no correlation between RBC content and RBC velocity (r2: frog 0.095, rat 0.002), both parameters contributing equally to variations in RBC supply rate. Temporal distributions of the three supply parameters demonstrate (1) the wide range of values present within each capillary, and (2) the symmetry of the distributions at high mean values versus a pronounced skewness as mean values approached zero. The wide range of mean values of each parameter, found in both species, suggests considerable 'spatial' heterogeneity of RBC supply among capillaries in resting muscle. Temporal heterogeneity of RBC supply rate to individual capillaries increased markedly as either velocity or content (or both) approached zero (overall mean coefficient of variation: frog, 62%; rat, 100%). Both species show remarkably similar rates of convective transport of RBCs per capillary in resting muscle (e.g. mean supply rates 3.2 and 6.2 RBCs/s in frog and rat, respectively).