Reactive oxygen species (ROS) are known to play an important role in the proliferation and viability of vascular smooth muscle cells. We have shown previously that treatment of fetal pulmonary arterial smooth muscle cells (FPASMC) with concentrations of 25 microM and higher of EUK-134, a superoxide dismutase/catalase mimetic, decreased cell viability via the induction of apoptosis. Here we demonstrate a dose-dependent decrease in serum-induced FPASMC growth at lower doses of EUK-134. This was due to the attenuation of FPASMC proliferation rather than the induction of apoptosis. Moreover, we found that the inhibition of FPASMC proliferation was observed using EUK-134 at concentrations as low as 5 microM. This inhibition of proliferation correlated with a 31% decrease in superoxide levels, as estimated using the oxidation of dihydroethidium. Flow cytometry revealed an increase in FPASMC in G2 after 24 h of exposure to 10 microM EUK-134. This was associated with a twofold increase in levels of the cell-cycle regulatory protein p21. This, together with our previous data, suggests that ROS levels determine the rate of FPASMC proliferation and, when below a threshold level, trigger apoptosis. Titration of ROS with antioxidants may help to prevent, or reverse, the vascular remodeling manifest in many cardiovascular disease states.