Velocities of an India ink front and of RBCs moving ahead of it were studied by intravital microscopy in capillaries of frog mesentery and skeletal muscle. Measurements were made during microperfusion of single vessels or groups of connected vessels in mesentery, and following intravenous ink injection in both tissues. The presence of ink did not appear to interfere with microvascular flow or vasomotion during the period of observation. On the average, the ink spearhead moved only slightly faster than the RBCs. There was substantial variation in relative velocities of RBCs in the same vessel and in the relative velocities of ink front and RBCs. The time course of ink filling showed substantial heterogeneity of flow in mesentery, more nearly uniform flow in skeletal muscle. Comparison of the measured velocity ratios of ink to RBCs with published observations on relative velocities of RBC to blood suggest that the advancing, apparently parabolic front of ink moves at less than twice the mean blood velocity. This is due in small part to diffusive dispersion of the ink particles in the laminar flow gradient, but more largely to stochastic dispersion of the front by interaction with RBCs and by displacement at branches.