The three dimensional association of endothelial vesicles must be a determinant in their role in transcapillary transport: Vesicles which shuttle materials across the capillary wall would constitute a discontinuous and nonhydraulically-conductive pathway. Transendothelial channels of fused vesicles would be true pores conducting materials in response to hydrostatic and osmotic pressure gradients across the capillary wall. Sessile, fused clusters of vesicles attached to the plasma membrane but not communicating across the endothelial cell would be inconsequential in transcapillary transport. In order to discern the three dimensional association of vesicles, we have employed high voltage electron microscopy to obtain stereopairs of thick sections through blood capillaries.Whole rats were perfuse-fixed with glutaraldehyde-tannic acid (both 1% w/v) via the left ventricle of the heart. The diaphragm muscle was removed and cut into small strips parallel to the muscle fibers. These were osmicated, dehydrated and embedded in Spurr's medium in an orientation such that cross sections of the capillaries could be taken.