The elements of the cell cycle regulatory machinery activated by the oncogenic form of Ras, [Lys61]N-Ras, have been analysed in NIH3T3 cells. We demonstrate that [Lys61]N-Ras expression is able to induce full cdk4 activation. As already reported, oncogenic Ras expression was sufficient to induce cyclin D1 and p21cip1 expression and their association with cdk4. Furthermore, serum-starved [Lys61]N-Ras NIH3T3 cells showed nuclear accumulation of cyclin D1 and cdk4 not observed in serum-starved NIH3T3 cells. This accumulation of cdk4 into the cell nucleus observed in serum-starved [Lys61]N-Ras NIH3T3 cells was inhibited by a microinjection of neutralizing anti-Ras antibodies. Thus, active [Lys61]N-Ras was a sufficient signal to induce nuclear accumulation of cyclin D1/cdk4, leading to its full activation. Transfection of [Lys61]N-Ras NIH3T3 cells with an inactive form of MEK or their treatment with PD 98059, showed that nuclear translocation of cdk4 was MEK dependent. Interestingly, cells constitutively expressing [Lys61]N-Ras did not inactivate pRb and did not proliferate in the absence of serum. This may be due to the fact that although association of cdk2 with cyclin E and the translocation of those complexes to the nucleus were achieved, [Lys61]N-Ras expression was not sufficient to induce cdk2 activation. The high levels of p27(kip1) that were found in cyclin E/cdk2 complexes may be responsible for the inability of oncogenic Ras to activate this kinase. In consequence, oncogenic alterations that lead to a decrease in p27kip1 bound to cyclin E may cooperate with Ras to induce full cdk2 activation, pRb inactivation and thus cell proliferation.