The chromosomes were studied in 16 metastatic Rous (RSV—SR) rat sarcomas. About 40 % of the tumours had a normal diploid stemline and of these tumours about 70 % had 1 or 2 hyperdiploid sidelines. Among the heteroploid metastases, hyperdiploidy accounted for 2/3 and pseudodiploidy for 1/3 of the stemlines. The chromosome variation in the development of the heteroploid stem- and sidelines was non-random and had the same characteristics as previously found in primary Rous rat sarcomas. There was a close karyotypic relation between the primary sarcomas and their metastases. The essential difference between them was an accelerated chromosomal progression in the secondary tumours. This progression was associated with a decrease in the histologic maturity of the tumours.
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