A primate toxicity study was performed to test the hypothesis that BUdR does not increase the likelihood of unilateral or bilateral central nervous system damage secondary to radiation therapy. BUdR, a halogenated pyrimidine analog is incorporated into DNA of dividing cells and sensitizes them to radiation. It is best given unilaterally, intra-arterially by continuous infusion because of its regional advantage (Rd) estimated to be between 11 and 16. Six rhesus monkeys were implanted with a Model 400 Infusaid pump perfusing the right internal carotid artery. Three of the six monkeys received intra-arterial (IA) BUdR infusion plus whole brain external beam radiation (6,000 R over 6 weeks) and three received radiation alone. The two BUdR treated animals completing radiation developed symmetric bilateral high signal aberrations on MRI in the frontal, parietal, and occipital centrum semiovale and corona radiata at nine months. At autopsy, confluent microinfarcts were found to correspond topographically to the MRI abnormalities. In the radiation alone group, two animals had normal MRI and autopsy while the third animal had bilateral MRI high signal aberrations develop sequentially with corresponding microinfarcts at autopsy. These changes were greater in severity than those seen in the BUdR treated animals. We support previous evidence that there is differential intraspecies sensitivity to radiation. We find that BUdR produces no unilateral potentiation of radiation toxicity.