In an attempt to find a strategy to modulate the proliferation of vascular endothelial cells, we examined whether constitutive activation of proto-oncogen protein p21 (Ras) induced the reentry of confluent human umbilical vascular endothelial cells (HUVECs) into the S phase. When an adenovirus construct expressing a constitutively active Ras mutant (Ad/RasG12V) was infected into HUVECs, their morphology changed strikingly and they appeared to be transformed. However, Ad/RasG12V-infected HUVECs did not enter the S phase, as determined by assessing 3H-thymidine incorporation. In accordance with the above results, the expression of cyclin A both at the transcript and protein levels did not increase in Ad/RasG12V-infected HUVECs relative to that in control cells, although the expression of cyclin D1 was induced in Ad/RasG12V-infected cells. Interestingly, the expression of the cyclin-dependent kinase (CDK) inhibitor p21cip1 was remarkably increased while that of p27kip1 did not decrease in Ad/RasG12V-infected HUVECs. Furthermore, CDK2 activity was not induced in Ad/RasG12V-infected HUVECs. These results suggested that the constitutive activation of Ras promoted the reentry of confluent HUVECs in the G0 phase into the G1 phase, but not into the S phase. The results also indicated that the constitutive activation of Ras might have induced the persistent expression of p21cip1 and p27kip1, and that this induction of p21cip1 and p27kip1 expression possibly caused the cell cycle arrest at the G1 phase.