A re-evaluation on the glomerular microcirculatory unit was made by a simple corrosion casting technique coupled with SEM. The morphological details of minute vessels forming the microcirculatory unit of the glomerulus were described. In addition, new information concerning the major difference between afferent and efferent arterioles was documented. Using a low viscosity plastic as a casting medium, the afferent arterioles were readily distinguishable from the efferent arterioles. A single row of ovoid endothelial impressions was evident at the hilus of the afferent arteriole, suggesting the occurrence of a sphincter on this vessel but not on the efferent vessels which did not show impressions of endothelial cells.