Serum stimulation of quiescent fibroblasts leads to a transient induction of the transcription factor egr-1. However, the induction of egr-1 by v-src was found to be sustained rather than transient. The proto-oncogene fos has been reported to be co-regulated with egr-1 and to repress serum-induced egr-1 expression. We found that c-fos prevents v-src-induced gene expression regulated by the egr-1 promoter. Thus, the sustained induction of egr-1 by v-src could be explained by a lack of c-fos induction by v-src. Consistent with this hypothesis, egr-1 and c-fos were co-induced by serum, but not by v-src, in Balb/c 3T3 cells; v-src did not induce c-fos expression in these cells. We propose that sustained expression of egr-1 induced by v-src in Balb/c 3T3 cells is due to a lack of c-fos down-regulation of egr-1.