Hemorrhage in the midbrain and/or pons in patients is often associated with increased metabolism, resulting in hyperthermia. We have recently reported that hyperthermia develops in anesthetized rats following prepontine knife-cuts or procaine microinjections into the midbrain or upper pontine region. It was concluded that the hyperthermia in the animals was caused by the removal of a tonic inhibitor mechanism of heat production that exists in the lower midbrain. The present paper proposes a new hypothesis that the hyperthermia in patients with brainstem hemorrhage is caused by disinhibition of heat production due to the release of such a lower-midbrain mechanism.