The vagal afferent innervation of the rat pancreas was investigated by the use of retrograde axonal transport of horseradish peroxidase (HRP) injected into the pancreas through the common bile duct. Many labeled neuronal somata were found in the right and left nodose ganglia. The size distribution of the labeled neuronal population in the nodose ganglia was examined and compared with that obtained from rats pretreated with capsaicin. In rats treated neonatally with capsaicin the smallest neurons (up to 200 micron2) were completely absent and those belonging to the second and third classes (200-300, 300-400 micron2) were reduced in both vagi.