To examine direct influences of capillary blood flow on its permeability of water-soluble substances, we measured the local capillary permeability in the rabbit tenuissimus muscle at various capillary blood flow levels by the use of microscopic tissue clearance method. After staining the muscle with Cr-EDTA (M.W.: 341) as the test tracer by suffusing its solution around the tissue, subsequent concentration change in the tissue due to the tracer washout by capillary blood flow was measured from the intensity change on the TV monitor through the vital-microscope. The decay constant of obtained tissue clearance curve has been theoretically predicted to be equal to the local capillary permeability surface area product (PS) per unit tissue volume (Vt). The local capillary permeability (P) could be quantified by calculating the capillary surface area (S) from the perfused capillary density in the visualized microscopic field. The calculated values of P and PS/Vt in the high flow state were 6.00 +/- 0.70 (X 10(-6) cm/s) and 5.85 +/- 0.77 (X 10(-4)/s), respectively, which were significantly larger than those in the low flow state (p < 0.01). It was suggested that there was a mechanism in the red cell passage through the capillary which facilitated the substance exchange across the capillary wall.