The Ras family of GTP-binding proteins are key transducers of extracellular signals, particularly through the mitogen-activated protein kinase (MAPK) pathway. Constitutively active forms of Ras are found in a variety of tumours, suggesting an important role for this pathway in cancer. Here we report that initial cellular exposure to oncogenic Ras chronically activated the MAPK pathway in the cytoplasm, but transiently activated the same pathway in the nucleus. Nuclear-activated extracellular signal-regulated kinase (ERK) was rapidly dephosphorylated, with consequent short-term activation of the Elk-1 transcription factor and expression of the c-fos gene. Additional experiments suggested that the regulatory mechanism involved requires the calcium-dependent protein phosphotyrosine phosphatase MAPK phosphatase-1 (MKP-1). This is the first report on the ability of Ras, in the absence of growth factors, to transiently activate the MAPK pathway in the nucleus and show an involvement of MKP-1 in nuclear ERK2 regulation. In addition we show that transient activation of the MAPK pathway is sufficient to drive chronic cell-cycle progression. We conclude that, whereas the MAPK pathway is necessary to initiate cellular proliferation and transformation, the transient nature of the MAPK pathway activation suggests the involvement of additional signalling pathway(s) regulated by Ras.