Chromosomal constitution of 7 rat glial tumors induced by methylnitrosourea (MNU) and ethylnitrosourea (ENU) was investigated. Cells from rat glial tumors showed preferentially complete monosomy for # 3 chromosomes and partial monosomy for # 2 chromosomes. Numerical and morphological changes in # 11 and # 12 chromosomes were also observed. These chromosome aberrations have no relation with histological classification of experimental rat gial tumors. One # 3 chromosome missing in neolplastic glial tumors was constantly observed through a long term culture and had no relation with the appearance of marker chromosome. These chromosome changes were compared with the preferential involvement of # 1 and 2 chromosomes in fetal brain cells of rats shortly after a transplacental administration of ENU.