Brain stem auditory evoked potentials (BAEPs) were studied as a criterion for evaluating hypothermic effects on the brainstem in rats. Eleven adult male Long-Evans rats were cooled slowly from 37 degrees C to 25 degrees C by an ice pad. BAEPs were recorded at intervals of every 1 degree C of change, and both central and peripheral acoustic conductions were assessed. As the body temperature decreased, latencies significantly increased, with the most prominent effect at the higher brainstem level. Interpeak latencies were prolonged also, while the reverse occurred with warming. These data agree with previous studies in other animals and men. However, during analysis of the effects of hypothermia on the brain stem auditory pathway, it was shown that these hypothermic effects were equally distributed throughout the whole pathway in the present study, rather than being emphatic in the proximal portion only. Greater prolongation of latencies in the proximal portion was caused by a cumulative effect from the distal peaks. Recovery was attained completely after rewarming and subsequent survival. Under extreme hypothermia, BAEPs disappeared gradually with apnea and cardiac arrest; however, resuscitation and rewarming saved the rats. In addition to wishing to make conventional observations, the present study sought to recognize the section of the acoustic pathway most sensitive to hypothermic effects and interpret the outcome after hypothermia treatment.