Chromosomes were studied in 9 primary and 14 transplanted rat tumors, including 9 N-nitrosobutylurea (NBU)-induced leukemias, 6 Gross virus-induced lymphosarcomas, 3 Friend virus-induced lymphosarcomas, 4 Rauscher virus-induced lymphosarcomas, and 1 Dunning spontaneous leukemia. Among these, 8 primary and 6 transplanted tumors showed a diploid karyotype. One NBU-induced primary erythroblastic leukemia also had a diploid stemline and several hypotetraploid variants. The remaining 8 transplanted tumors were pseudodiploid, hyperdiploid or hypodiploid, or a mixture of diploid and heteroploid; 1 exceptional case had a hypotetraploid stemline. The banding karyotype analysis revealed that the diploid tumors were indistinguishable from the normal rat somatic complex, except 1 case with minute structural changes. Other minor structural changes and markers were detected in some heteroploid tumors; the changes were not similar except for 2 transplanted Rauscher lymphomas that showed either complete or partial trisomy for #1 chromosomes. Certain karyotypic transitions were observed in some tumors during prolonged in vivo or in vitro passages as well as in the metastatic process, but these changes were usually confined within the diploid range. By contrast, the Dunning leukemia, the oldest tumor in this series, exhibited drastic structural rearrangements with greater karyotypic variations. From these observations we concluded that the stemline cells of the rat leukemias and lymphomas were most stable at the diploid level.