In Chinese hamster embryo fibroblasts (IIC9 cells), platelet-derived growth factor (PDGF) stimulated mitogen-activated protein kinase/extracellular-signal-regulated kinase (MAP kinase/ERK) activity, but not that of c-jun N-terminal kinase (JNK), and induced G₁ phase progression. ERK1 activation was biphasic and was sustained throughout the G₁ phase of the cell cycle. PDGF induced cyclin D1 protein and mRNA levels in a time-dependent manner. Inhibition of PDGF-induced ERK1 activity by the addition of a selective inhibitor of MEK1 (MAP kinase kinase/ERK kinase 1) activation, PD98059, or transfection with a dominant-negative ERK1 (dnERK^-) was correlated with growth arrest. In contrast, growth was unaffected by expression of dominant-negative JNK (dnJNK^-). Interestingly, addition of PD98059 or dnERK^-, but not dnJNK^-, resulted in a dramatic decrease in cyclin D1 protein and mRNA levels, concomitant with a decrease in cyclin D1–cyclin-dependent kinase activity. To investigate the importance of sustained ERK1 activation, ERK1 activity was blocked by the addition of PD98059 throughout G₁. Addition of PD98059 up to 4 h after PDGF treatment decreased ERK1 activity to the levels found in growth-arrested IIC9 cells. Loss of cyclin D1 mRNA and protein expression was observed within 1 h after inhibition of the second sustained phase of ERK1 activity. Disruption of sustained ERK1 activity also resulted in G₁ growth arrest. These data provide evidence for a role for sustained ERK activity in controlling G₁ progression through positive regulation of the continued expression of cyclin D1, a protein known to positively regulate G₁ progression.