Central neurotoxicity produced in rats by daily administration of 300 mg/kg of misonidazole (MISO) 5 times/week for 4-5 weeks (total dose = 6.0 gm/kg) was evaluated weekly wit brain stem auditory evoked potentials (BAEPs). Compared to untreated control rats, all treated rats had a prolongation of the I-IV interpeak latency (p less than 0.005) at a mean of 13.2 +/- 2.7 days at a cumulative dose of approximately 4.0 gm/kg of MISO per rat. In some rats, the I-III and I-II interwave latencies were prolonged and waves III and IV were lost. Control rats did not show any significant alteration in BAEP latency or amplitude. Histopathologic examination of the brain stems of treated rats showed that necrotic lesions were present primarily in the nuclei of the tegmentum of the fourth ventricle, with scattered nuclear involvement in the cerebellar roof nuclei, inferior olive, and nucleus of the spinal tract of the trigeminal nerve. The cerebral cortex appeared to be normal in all treated rats. Changes in BAEPs caused by central neurotoxicity correlated with the histopathologic findings. We conclude that BAEPs are a sensitive method for evaluating MISO central neurotoxicity in the rat model.