Glomerular permselectivity characteristics were studied in the Sprague-Dawley rat in vivo and in the isolated rat kidney perfused with an erythrocyte-free Krebs-Henseleit buffered 5% albumin solution (IPK). IPK permselectivity in vitro, assessed by fractional clearances of neutral dextran (FCND) and dextran sulfate (FCDS) with molecular radii 18-43 A, was essentially similar to that of the Sprague-Dawley rat in vivo. The negative charge barrier of the IPK glomerular filter was intact [e.g., FCND of 36 A = 0.10 +/- 0.01 (SE) vs. FCDS of 36 A = 0.01 +/- 0.00 (P less than 0.01)]. Dextrans of an intermediate size (26-34 A) had lower fractional clearances in the IPK than in vivo [e.g., FCND of 30 A in IPK = 0.23 +/- .04 vs. FCND of 30 A in vivo 0.40 +/- 0.01 (P less than 0.01)]. This decreased clearance of dextrans of an intermediate molecular size is predicted by pore theory, since the IPK has an increased afferent glomerular plasma flow rate. As glomerular permselectivity characteristics in the IPK simulate in vivo characteristics, such preparations are suitable in vitro models in which to study factors that modulate permselectivity. The demonstration that the glomerular filter in the IPK has a normal negative charge barrier indicates that the increased protein excretion in IPK systems cannot be attributed to abnormalities of this component of the filtration barrier.