Stimulation of mitogenic signaling pathways results in transient activation of the extracellular signal-regulated kinase (ERK) subfamily of mitogen-activated protein kinases (MAPK) in normal cells. We demonstrate here that activation of ERKs in response to serum or phorbol ester stimulation was markedly repressed in three different rodent fibroblast cell lines stably transformed by v-Src. Activation of the MAPK/ERK kinase (MEK) was also repressed in v-Src-transformed cells, indicating that the repression occurs upstream of ERK. Consistent with repression occurring predominantly at the level of MEK, the phosphatase inhibitor orthovanadate could restore ERK activation to a limited extent in some but not all v-Src-transformed cell lines. A similar repression of ERK activation was observed in v-Ras- and v-Raf-transformed cells. In addition, ERK activity was not constitutively elevated in exponentially growing cells transformed by v-Src, v-Ras, or v-Raf as compared with normal cells. These results establish that the ERK activation pathway is repressed in rodent fibroblasts stably transformed by viral oncoproteins that chronically stimulate receptor tyrosine kinase signaling pathways. Furthermore, our findings suggest that elevated ERK activity above basal levels is not required for maintaining cell transformation by these oncoproteins. Taken together, these results indicate that ERK signaling pathways are subject to negative feedback regulation upstream of ERK as a consequence of oncogenic transformation.