Experimental allergic neuritis (EAN) induced in the Lewis rat by the adoptive transfer of a P2-specific T cell line (AT-EAN) is considered an animal model of Guillain-Barré syndrome. It is not yet known whether AT-EAN is inducible at early stages in the development of the peripheral nervous system (PNS) or whether disease activity is modified because of immaturity of either the nervous system or the immune system. We therefore compared the susceptibility of neo-natal and adult Lewis rats to AT-EAN induced by the adoptive transfer (intraperitoneally) of 106 activated P2-specific T cells. P2 antigen was already present in 7 day old Lewis rats and P2-specific T cell transfer into 3-day-old rats induced clinical disease associated with an inflammatory response (sciatic nerves and spinal ganglia). In injected newborn rats we observed local activation of mast cells, infiltration of the PNS by inflammatory cells, and induction of Ia antigen expression in Schwann cells. Unlike in adults, segmental or paranodal demyelination despite occasional nerve fiber degeneration did not occur. However, the difference between newborn and adult rats could not be ascertained statistically because of the relative rarity of the lesions, their focal character, the admixture of fiber demyelination and degeneration, and most importantly, size differences of the myelinated fibers, which result in a large developmental decrease in fiber density in adults compared to newborns.