The size and charge-selective properties of the glomerular barrier are partly controversial. Glomerular sieving coefficients (θ) for proteins have rarely been determined noninvasively before in vivo. Therefore, θ was assessed vs. glomerular filtration rate (GFR; ⁵¹Cr-EDTA clearance) in intact rats for radiolabeled myoglobin, κ-dimer, neutral horseradish peroxidase (nHRP), neutral human serum albumin (nHSA), and native albumin (HSA). To obtain θ, glomerular tracer clearance, assessed from the 7- to 8-min kidney uptake of protein, was divided by the GFR. The data were fitted with a two-pore model of glomerular permeability, where the small-pore radius was 37.35 ± 1.11 (SE) Å, and the “unrestricted pore area over diffusion path length” (A ₀/ΔX) 1.84 ± 0.43 · 10⁶ cm. Although seemingly horizontal for nHRP and nHSA, the log θ vs. GFR curves showed slightly negative slopes for the proteins investigated in the GFR interval of 2–4.5 ml/min. Strong negative (linear) correlations between (log) θ and GFR were obtained for myoglobin (P = 0.002) and HSA (P = 0.006), whereas they were relatively weak for nHRP and nHSA and nonsignificant for κ-dimer. θ for nHSA was markedly higher than that for HSA. In conclusion, there were no indications of increases in θ vs. GFR, as indicative of concentration polarization, for the proteins investigated at high GFRs. Furthermore, the glomerular small-pore radius assessed from endogenous (neutral) protein sieving data was found to be smaller than previously determined using dextran or Ficoll as test molecules.