Accumulating evidence suggests that T cells and autoantibodies reactive with myelin oligodendrocyte glycoprotein (MOG) play a critical role in the pathogenesis of multiple sclerosis (MS). In the present study, we have tried to elucidate the pathomechanisms of development and progression of the disease by analysing T cells and autoantibodies in MOG-induced rat experimental autoimmune encephalomyelitis (EAE), which exhibits various clinical subtypes mimicking MS. Analysis using overlapping peptides revealed that encephalitogenic epitopes resided in peptide 7 (P7, residue 91-108) and P8 (residue 103-125) of MOG. Immunization with MOGP7 and MOGP8 induced relapsing-remitting or secondary progressive EAE. T cells taken from MOG-immunized and MOGP7-immunized rats responded to MOG and MOGP7 and sera from MOG-immunized rats reacted to MOG and MOGP1. Significant epitope spreading was not observed at either T-cell or antibody levels. Interestingly, sera from MOGP7-immunized rats with clinical signs did not react to MOG and MOG peptides throughout the observation period, suggesting that disease development and relapse in MOGP7-induced EAE occur without autoantibodies. However, MOGP7 immunization with adoptive transfer of anti-MOG antibodies aggravated the clinical course of EAE only slightly. Analysis of antibodies against conformational epitope (cme) suggests that anti-MOG(cme) may play a role in the pathogenicity of anti-MOG antibodies. Collectively, these findings demonstrated that relapse of a certain type of MOG-induced EAE occurs without autoantibodies but that autoantibodies may play a role in disease progression. Relapses and the progression of MS-mimicking EAE are differently immunoregulated so immunotherapy should be designed appropriately on the basis of precise information.