Somatosensory evoked potentials (SEP) have been applied to many men and several animals with cerebral ischemia. But in the Mongolian gerbil, one of the most common models of cerebral infarction, the wave form and the latency of SEP have been studied by few authors. In this report, we studied the effects of hyperthermia on SEP in the gerbil. Six gerbils weighting 40 to 50g were anesthetized and heated with ultra-red ray. Brain and rectal temperatures were continuously monitored. The brain temperature changed from 35 degrees C to 42 degrees C during heating, then returned to 37 degrees C with cooling. Typical SEP peaks have been labelled according to their positive or negative character and their sequential latency determination. The latency of the major positive voltage deflection, P1, was 9.97 +/- 9.94 ms at 37 degrees C. The negative deflection before P1 has been labelled as N0 and the negative waves after P1 and N1 and N2. Peak latency of SEP was significantly reduced gradually during hyperthermia, and increased gradually with recooling. The correlation between the latency of SEP and the brain temperature was very close. Our results emphasize the importance of measurement of SEP during hyperthermia as a monitor of the brain temperature and during experiment for maintenance of the body temperature at 37 degrees C.