Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) is widely used to evaluate tumor permeability, yet measurements have not been directly validated in brain tumors. Our purpose was to compare estimates of forward leakage Ktrans derived from DCE-MRI to the estimates K obtained using [14C]aminoisobutyric acid quantitative autoradiography ([14C]AIB OAR), an established method of evaluating blood-tumor barrier permeability. Both DCE-MRI and [14C]AIB OAR were performed in five rats 9 to 11 days following tumor implantation. Ktrans in the tumor was estimated from DCE-MRI using the threeparameter general kinetic model and a measured vascular input function. Ki was estimated from OAR data using regions of interest (ROI) closely corresponding to those used to estimate Ktrans. Ktrans and Ki correlated with each other for two independent sets of central tumor ROI (R = 0.905, P = .035; R = 0.933, P = .021). In an additional six rats, Ktrans was estimated on two occasions to show reproducibility (intraclass coefficient = 0.9993; coefficient of variance = 6.07%). In vivo blood-tumor permeability parameters derived from DCE-MRI are reproducible and correlate with the gold standard for quantifying blood tumor barrier permeability, [14C]AIB OAR.