C-fos expression was used to determine areas at which neurons were excited by electrical stimulation of the tooth pulp in the cat forebrain. Tooth pulp stimulation (0.2msec duration, twin pulse) was delivered at 1 Hz under pentobarbital sodium anesthesia (35㎎/kg) and the intensity was maintained at 3 times the threshold for the jaw-opening reflex (200-600μA). With a survival time of 1.5 hrs following the start of stimulation, cats were perfused with buffered paraformaldehyde. Brain slices were incubated with polyclonal rabbit antibody against the Fos protein that originated in c-fos and was processed according to the peroxidase-antiperoxidase (PAP) method. In the pentobarbital sodium-inlected (35㎎/0.7ml/kg, 2 hrs before sacrifice) group, Fos-positive neurons were found bilaterally in the prelimbic, infralimbic, prepiriform, perirhinal, periamygdaloid and entorhinal cortices, lateral habenular (HbL), thalamic and hypothalamic paraventricular nuclei, supraoptic nucleus (SON), infundibular nucleus and anterior preoptic area. Furthermore, tooth pulp stimulation resulted bilaterally in Fos expression in the granular insula and increased the number of Fos-positive neurons to 368% in SON and 280% in HbL. In addition, the increases were inhibited by morphine administration (2㎎/kg, i.p.). These findings suggest that SON and HbL are involved in defensive mechanisms to noxious stimulation through the release of vasopressin and antinociception.