Schmidt-Ruppin strain of Rous Sarcoma was inoculated intracerebrally into 27 newborn beagle dogs. Fourteen days after viral inoculation, 13 of the dogs were given intravenous BCNU (1 mg/kg). The other 14 were given the same volume of intravenous saline in a randomized, double-blinded fashion. Ninety percent of all dogs developed intracranial tumors. Radionuclide (mercury 197) brain scans were done on each dog at 2-week intervals. Median survival was 113 days in the BCNU group and 115 days in the placebo group (P > .99). Unequivocally positive radionuclide brain scans were detected in 5 dogs treated with BCNU and in 2 of the controls. There were no gross or microscopic differences at autopsy between treated and nontreated animals. BCNU, as given in this animal brain tumor model, did not demonstrate any oncolytic effect. An improvement in sequential brain scans was detected in 2 other dogs in response to Dexamethasone, which was given in a double-blinded, cross-over controlled fashion. Computerized tomography clearly demonstrated the tumor in two cases.