Synaptonemal complexes revealed mutagen-induced effects in germ cell meiotic chromosomes. Male mice were irradiated by 4 Gy X-rays and spermatocytes were harvested at different time after exposure. Various form of synaptonemal complex aberrations were observed, such as breaks, rings, rearrangement which were consistent with those aberrations observed in metaphase I chromosome by conventional cytogenetic analysis. Some novel forms of synaptonemal complex aberrations were observed. The relationship between synaptonemal complex aberration and chromosome aberration was discussed.