This study investigated possible collateral C-fiber innervation between the pulps of rat molars by assessing neurogenic inflammation (NI) induced by the C-fiber excitant mustard oil (MO). MO was applied to the pulp of the left mandibular first molar in two groups of rats: group 1, guanethidine sympathectomized rats (to dismiss sympathetic activation by MO); and group 2, unsympathectomized rats. A third group of unsympathectomized rats (group 3) had saline applied to the pulp of the left mandibular molar and served as a MO control. The NI-related plasma extravasation was examined in these teeth and in the remaining left mandibular teeth by a spectrophotometric analysis of extravasated plasma protein bound to Evans' Blue (EB) dye. The collateral innervation pattern was inferred from the NI pattern. EB concentrations were measured in the left mandibular teeth and the corresponding contralateral teeth, and expressed as a ratio. Statistical analysis of the data revealed significant differences in EB ratios in the first, second, and third molars between groups 1 and 3. This result suggests collateral C-fiber innervation exists within the pulps of molar teeth in the same dental quadrant. No difference in EB ratios was noted in the first and second molars between groups 1 and 2. Therefore, sympathetic efferents have no apparent effect on the degree of MO-induced NI.