The glomerular barriers to water and macromolecular movement were examined 2 and 24 h after the administration of a monoclonal antibody (mAb) specific to an antigen located on the epithelial slit diaphragm and the external aspect of the glomerular basement membrane. By micropuncture techniques 2 h after mAb administration, single-nephron GFR (SNGFR) and plasma flow were unchanged but the glomerular capillary hydrostatic pressure gradient and glomerular capillary hydrostatic pressure were increased and the glomerular ultrafiltration coefficient (LpA) decreased to values that were 50% of the normal control. There was no increase in urinary protein excretion at 2 h. However, at 24 h after mAb, nephron plasma flow (SNPF) and SNGFR increased and the glomerular ultrafiltration coefficient returned to values indistinguishable from the normal control. At 24 h, there was a marked increase in protein excretion. The administration of meclofenamate decreased values for SNGFR and SNPF to normal. Immunoglobulin G was exclusively bound to glomerular capillary walls in a linear or continuous fashion at 2 h, but in a discontinuous, granular pattern at 24 h. These studies suggest that after mAb, important limiting glomerular barriers for hydraulic conductivity and protein excretion reside on the epithelial aspect of the glomerular capillary basement membrane, specifically at the level of the slit diaphragm. Studies also suggest that alterations in glomerular capillary hydraulic conductivity can be effectively separated from increases in macromolecular permeability.