The evaluation of candidate optical contrast agents for brain tumor delineation in ex vivo models may not accurately predict their activity in vivo. This study describes an in vivo model system designed to assess optical contrast agents for brain tumor delineation. The brain tumor window (BTW) model was created by performing biparietal craniectomies on 8-week-old Sprague-Dawley rats, injecting 9L glioma cells into the cortex and bonding a cover slip to the cranial defect with cyanoacrylate glue. Tumor growth was followed serially and occurred in an exponential fashion. Once tumors on the cortical surface achieved a 1mm radius, intravenous contrast agents were injected while the appearance of the cortical surface was recorded. Computerized image analysis was used to quantitatively evaluate visible differences between tumor and normal brain. Tumor margins became readily apparent following contrast administration in the BTW model. Based on red component intensity, tumor delineation improved fourfold at 50 min post-contrast administration in the BTW model (P<0.002). In summary, window placement overlying an implanted glioma is technically possible and well tolerated in the rat. The BTW model is a valid system for assessing the in vivo activity of optical contrast agents.