Analysis of chromosome numbers and flow cytometric measurements of relative DNA content have been performed on four neoplastic neurogenic rat cell lines obtained from fetal BD IX-rat brain cells exposed to N-ethyl-N-nitrosourea (EtNU) in vivo, which sequentially underwent neoplastic transformation in cell culture. In vitro, the cell lines showed varying degrees of ploidy (ranging from near-diploid to near-hexaploid values) and chromosome aberrations. When grown in vivo as solid tumours in BD IX-rats and in thymus deficient nude mice, or in intraperitoneal diffusion chambers in rats, the near diploid cell lines were stable. In the lines with higher ploidies the proportion of cells with near-diploid values increased. Upon retransfer to cell culture, the cell lines gradually changed to the original in vitro pattern. However, in some cases an elevated near-diploid fraction persisted during culture after retransfer. Despite the selection of malignant cells with different ploidy in vivo and in vitro, their ultrastructural morphology remained unchanged. Thus, in this system, near-diploid cells are favoured under conditions of in vivo growth.