Measurements of filtration coefficients (Lp) and osmotic reflexion coefficients (sigma) of single capillaries in the frog mesentery suggest that fluid flows through the walls of these vessels in two types of channel. One channel type (representing about 10% of Lp) appears to be exclusively available for water whereas the other channels appear available for both water and hydrophilic solutes though they severely restrict the passage of albumin (sigma = 0.81). The effects of proteins in the perfusate upon Lp and studies on the passage of ferritin into the surface vesicles of the endothelial cells, suggest that an important component of capillary permeability may reside in an endocapillary layer.