[en] Whole body gamma irradiation (4 Gy) of male laboratory rats, Rattus norvegicus, induced chromosomal damage and decrease of mitotic index in bone marrow cells which were investigated 0-1/2, 6, 24 and 48 hr. After treatment. Chromosomal aberrations observed consisted of chromatid breaks, centromeric attenuation, chromosomal translocations and rings. The intraperitoneal administration of imidazole at 0.35 mg/g body weight prior to irradiation exerted a definite protective character against radiation induced chromosomal aberration and affected the mitotic index of bone marrow cells