The expression of the proto-oncogenes c-fos, c-jun, jun B and jun D was monitored in quiescent C3H10T1/2 fibroblasts after stimulation with PDGF. The mRNA level of c-fos, c-jun and jun B, but not of jun D, was stimulated by PDGF. The inductions were abolished when genistein, a specific tyrosine protein kinase inhibitor, was added concomitantly with PDGF, a condition in which DNA synthesis is known to be inhibited. As already shown previously, treatment with PDGF and genistein for 4h followed by the replacement with fresh medium induces the progression of the cells through the G1 phase of their growth-division cycle, without phospholipase C activation. The removal of PDGF and genistein was accompanied by an important increase in c-fos, c-jun and jun B mRNA expression, which correlated with the entrance of cells into G1 phase. Thus, the proto-oncogene expressions induced by PDGF are also obtained in the absence of phospholipase C activation. This result also suggests that the mRNA levels of c-jun, jun B and to a lesser degree c-fos are positively regulated by tyrosine protein kinase activity, whereas jun D is negatively regulated.