To investigate the charge effect of the endothelial surface glycocalyx on microvessel permeability, we extend the 3-D model developed by Fu et al. (1994) for the interendothelial cleft to include a negatively charged glycocalyx layer at the entrance of the cleft. Both electrostatic and steric exclusions on charged solutes are considered within the glycocalyx layer and at the interfaces. Four charge density profiles are assumed for the glycocalyx layer. Our model indicates that the overall solute permeability across the microvessel wall including the surface glycocalyx layer and the cleft region is independent of the charge density profiles as long as they have the same maximum value and the same total charge. Based on the experimental data, this model predicts that the charge density would be 25-35 mEq/l in the glycocalyx of frog mesenteric capillaries. An intriguing prediction of this model is that when concentrations of cations and anions are unequal in the lumen due to the presence of negatively charged proteins, the negatively charged glycocalyx would provide more resistance to positively charged solutes than to negatively charged ones.