Distribution of velocities of erythrocytes dependent upon their distance from the vascular axis were studied in 25-40 mu large microvessels of the frog mesentery. The method of in vivo cinemicrography with subsequent analysis revealed that with a mean axial velocity of blood flow higher than 0.3-0.4 mm/sec the velocity profile was parabolic, similar to the laminar flow of Newtonian fluids and independent of the erythrocytes concentration in microvessels. With velocities less than 0.3-0.4 mm/sec a chaotic distribution of erythrocytes velocities occurred and the parabolic form of the velocity profiles was rarely observed;--actually only at low erythrocyte concentration (less than 10-11%) in microvessels. The results suggest that an increase in vascular resistance to blood flow must appear during a considerable slow-down of blood flow velocity in microvessels.