A thermocouple reentrant tube was stereotaxically implanted in the rostral brain stem of the golden hamster, Mesocricetus auratus. Brain temperature was continuously recorded while the hamster was permitted volitional running on an activity wheel. There was an immediate decrease in brain temperature at the start of running activity, reaching a mean of 0.49 degrees C below the prerunning level. Spontaneous or forced cessation of running activity was associated with a rapid recovery of brain temperature. The time course of brain cooling during exercise greatly differed from the exponential decay of brain temperature in hyperthermic and dead animals. Air flow through the nose may contribute to the maintenance of a low brain temperature because nasal blockade promotes an increase in brain temperature. Below an ambient temperature of 33 degrees C, the resting hamster maintains its brain temperature below deep-body (abdominal) temperature. Vinyl acetate casts of the arterial and venous systems revealed several potential sites for heat exchange that might account for brain cooling under resting and exercising conditions.