We previously isolated and characterized the structure of murine thymidine kinase (tk) genomic and cDNA sequences to begin a study designed to identify regions of the tk gene important for regulated expression during the transition of cells from G0 to a proliferating state. In this report, we describe the stable transfection of the cloned gene into L-M(TK-) cells and show that both thymidine kinase (TK) enzyme activity and DNA synthesis increase in parallel when transfectants in G0 arrest are stimulated by serum. To define promoter and regulatory regions more precisely, we have constructed a series of tk minigenes and have examined their expression in stable transfectants after serum stimulation. We have identified a 291-base-pair DNA fragment at the 5' end of the tk gene that has promoter function, and we have determined its sequence. In addition, we have found that DNA sequences which mediate serum-induced expression of TK are transcribed, since expression of the murine tk cDNA, fused to a promoter from either the murine tk gene, the simian virus 40 early region, or the herpes simplex virus tk gene, is stimulated by serum. Our constructs also reveal that the murine tk polyadenylation signal is not required for regulation, nor is most of the 3' untranslated region. RNA dot blot analysis indicates that murine cytoplasmic tk mRNA levels always parallel TK enzyme activity. Nuclear runon transcription assays show less than a 2-fold increase in transcription from the cloned tk gene in serum-stimulated transfectants, but an 11-fold increase in mouse L929 cells, which are inherently TK+. These results taken together suggest that the murine tk gene is controlled in serum-stimulated cells by a transcriptional mechanism influenced by DNA sequences that flank tk and also by a posttranscriptional system linked to gene sequences that are transcribed.