We have studied SV40 gene expression during transforming infection of Go-arrested primary mouse kidney cell cultures. The results show that both early and late viral genes are simultaneously expressed during a short restricted period of time. Synthesis of T-antigen is also time limited and precedes the first SV40-induced cellular DNA replication and mitosis. Cells in mitosis retained stable T-antigen which was redistributed and transmitted to daughter nuclei. Even though no reinitiation of viral transcription occurred after the first mitosis, several cycles of cellular DNA synthesis and mitosis was observed. These results suggest that over a several day period, stable T-antigen conferred a continuous mitotic stimulus to the infected cells.