Platelet-derived growth factor (PDGF)-induced Ras activation is required for G1 progression in Chinese hamster embryo fibroblasts (IIC9 cells). Ras stimulates both extracellular signal-related kinase (ERK) activation and RhoA activation in response to PDGF stimulation. Inhibition of either of these Ras-stimulated pathways results in growth arrest. We have shown previously that Ras-stimulated ERK activation is essential for the induction and continued G1 expression of cyclin D1. In this study we examine the role of Ras-induced RhoA activity in G1 progression. Unstimulated IIC9 cells expressed high levels of the G1 cyclin-dependent kinase inhibitor p27(KIP1). Stimulation with PDGF resulted in a dramatic decrease in p27(KIP1) protein expression. This decrease was attributed to increased p27(KIP1) protein degradation. Overexpression of dominant-negative forms of Ras or RhoA completely blocked PDGF-induced p27(KIP1) degradation, but only dominant- negative Ras inhibited cyclin D1 protein expression. C3 transferase also inhibited PDGF-induced p27(KIP1) degradation, thus further implicating RhoA in p27(KIP1) regulation. Overexpression of dominant-negative ERK resulted in inhibition of PDGF-induced cyclin D1 expression but had no effect on PDGF- induced p27(KIP1) degradation. These data suggest that Ras coordinates the independent regulation of cyclin D1 and p27(KIP1) expression by the respective activation of ERK and RhoA and that these pathways converge to determine the activation state of complexes of cyclin D1 and cyclin-dependent kinase in response to mitogen.