The present study deals with the influence of preferential κ-opioid agonist MR 2034 on experimental allergic encephalomyelitis (EAE). For this purpose, 9-week-old male Dark August rats were treated intraperitoneally with 0.2 mg/kg of MR 2034 as follows: (a) from the day of EAE induction until sacrifice; (b) from the day of EAE induction until the appearance of neurological signs, and (c) from the appearance of neurological signs until sacrifice. Repeated injections of MR 2034 given during the whole period of observation produced the most pronounced suppression of EAE clinical signs, histological lesions in the brain and spinal cord, and anti-myelin basic protein antibody production. These results suggest that κ-opioid receptors may be involved in the development of EAE.