Human and primate studies have demonstrated that performance of tasks that induce asymmetrical physiological activation of the cerebral hemispheres leads to a reduction of tympanic temperature (TT) ipsilateral to the most active hemisphere. It is possible that diseases that interfere in an asymmetrical fashion with the degree of cerebral activity cause similar TT changes. There are not, however, normative studies of the acceptable interaural difference in TT in normal subjects at rest. This study was done to establish normative values for interaural TT values measured by means of infrared tympanic thermometry in resting normal subjects not engaged in any specific task. TT values were measured in 47 normal volunteers (20 men and 27 women, aged 39.38+/-12.57 years old) at rest; mean interaural differences of TT were calculated. Mean right ear TT was 36.85+/-0.50 degrees C and mean left ear TT was 36.74+/-0.57 degrees C; these values are in agreement with those already reported in the literature. Mean interaural TT difference was 0.25 degrees C (SD 0.21 degrees C). These findings indicate that maximal normal values for interaural TT differences, with confidence levels of 99% and 95%, are, respectively, 0.88 and 0.67 degrees C. The value of interaural differences of TT as a marker of asymmetrical hemispheric activity in neurological patients will have to be established by additional studies.