Erythrocyte-containing capillaries were counted in dog gracilis muscles freeze-clamped at rest and after twitch contraction at 4/s. In each of 21 muscles, 6--8 blocks were examined at -70 degrees C without fixation or staining; 15 fields were counted per block. Frequency analysis of capillaries per field based on the negative binomial distribution indicated that capillary density at rest was controlled by arterioles. Active vasomotion of these arterioles was "switched off" within 5 s after onset of exercise. Capillary density was then determined passively by stochastic rheologic factors acting at the individual capillaries. Thus exercise changes the site and the mechanism of capillary control. Recruitment occurred first where capillary density was lowest, and was complete in 15 s; this greatly decreased the heterogeneity of capillary spacing. Mean capillary density increased 1.5- to 3-fold, whereas flow increased almost 7-fold. Calculated mean velocity and mean transit time of erythrocytes in capillaries were 1.1 mm/s and 920 ms at rest and 4.2 mm/s and 215 ms after 3 min of exercise.