We have analyzed the expression of the ras gene family during the transit through quiescence-proliferation. H-, K- and N-ras steady-state mRNA levels and total Ras protein levels did not change significantly when NIH3T3 cells were made quiescent by density arrest in the presence of 10% calf serum. By contrast, levels of ras mRNAs in cells that had been made quiescent by serum deprivation were lower than those in growing cells. An induction of H-, K- and N-ras mRNA levels (3- to 5-fold) was detected in these cells after serum addition. This induction was maximal around 8 h after serum addition for the three ras genes. Like the early-response genes, ras induction was not dependent on protein synthesis; but in contrast to these genes, ras mRNAs showed long half-lives (5-7 h) in NIH3T3 cells. Up-regulation of ras genes by serum was also observed in human primary fibroblasts, indicating that this may be a general effect in mammal cells. We obtained stable transfectants in NIH3T3 cells with the oncogenic N-ras. In these cells, expression of the transforming gene is also induced by serum, and the expression of the transfected N-ras gene did not modify the response to serum of endogenous H-, K-, and N-ras genes. The regions of murine N-ras gene responsible for serum inducibility seem to be intragenic because N-ras up-regulation occurred in cells transfected with a gene construct lacking the sequences upstream from the first exon.(ABSTRACT TRUNCATED AT 250 WORDS)