Fluorescence labeled erythrocytes are suitable markers for red cell velocity and red cell flux through glomerular capillaries in incident light fluorescence microscopy. Mean velocity of the labeled cells was measured from a video recording with a microprocessor using a modified dual slit photometric technic. Red cell flux through the capillary was derived from the labeled cell frequency in capillary flow and systemic concentration of the fluorescent cells. The capillary diameters could be determined after staining plasma with fluorescence labeled dextran. The dynamic hematocrit (HMICRO) was calculated by the ratio of red cell flux to the product of red cell velocity and luminal cross section and varied between +28 and -51% from the corresponding systemic value.