The sensory cells of the nodose and jugular ganglia of the rat have been quantitatively evaluated in longitudinal paraffin sections. The right vagal ganglia contain significantly more neurons than the left, particularly neurons with somata having sectioned areas 200-400 microm2 and longest diameters 15-25 microm. Such neurons appear to be homogenously distributed because sections of the right vagal ganglia did not show specific areas of neuronal density compared with those of the left. Neonatal capsaicin treatment reduced the number of neurons in both the left and right ganglia to about 30% of controls. Capsaicin destroyed neurons with sectioned areas of 100-600 microm2 and longest diameters of 15-35 microm, but had no statistically significant effects on larger neurons.