The surface microstructure of fetal rat brain cells undergoing neoplastic transformation in long-term cell culture after a single transplacental pulse of 75 microgram N-ethyl-N-nitrosourea/g body weight to the fetal (18th day of gestation) BD IX rat was investigated by scanning electron microscopy. After about 3 weeks of culture, N-ethyl-N-nitrosourea-pretreated fetal rat brain cells showed focal proliferation of neural cells on an underlayer of flat, epithelioid cells. The neural cells exhibited varying forms of numerous dorsal ruffles and an increased number of other surface microprojections. Between the 40th and the 100th day, nodules of bipolar and multipolar neural cells were observed with a complex surface microstructure including many blebs and ruffles and an increased number of microvilli. After 100-210 days, more rapidly proliferating, morphologically altered cells formed "piled-up" foci, which resulted in a homogeneous population of cells with numerous long microvilli, large ruffles, and blebs over the whole surface. The cells retained the same altered surface structure until tumorigenicity after reimplantation into the syngeneic host was first observed (approximately 273 days). Surface alterations characteristic of the neoplastic cells were thus observable more than 100 days before the cells became tumorigenic.