Experimental allergic neuritis (EAN) was induced in Lewis rats aged 4 months by the inoculation of whole bovine dorsal root with Freund's complete adjuvant. Prolonged follow-up demonstrated that a relapsing course is a regular feature of the disorder in animals at this age. Although the initial disease episode was the most severe, clinical recovery from subsequent relapses was less satisfactory, this probably being related to persistent morphological abnormalities in the peripheral nervous system. Antecedent thymectomy, splenectomy, or the two combined, had little effect on the clinical course of the disorder, apart from reducing the duration of relapses. This was only statistically significant following combined thymectomy/splenectomy. Histological abnormalities, however, tended to be less severe in the operated as compared with normal control or sham-operated animals with EAN. The animals must have attained an immunocompetent state at the time of thymectomy and/or splenectomy. The capacity to develop EAN presumably resides in the draining lymph nodes and the occurrence of relapses is due to the continuing presence of antigen at the injection sites.