An emerging body of evidence is accumulating to suggest that in vivo formation of free radicals in the vasculature, such as peroxynitrite (ONOO-), and programmed cell death (i.e., apoptosis) play important roles in vascular diseases such as atherosclerosis, hypertension, and restenosis. The present study was designed to determine whether primary rat aortic smooth muscle cells (SMCs) undergo apoptosis following treatment with ONOO-. Direct exposure of primary rat aortic SMCs to ONOO--induced apoptosis in a concentration-dependent manner, as confirmed by means of quantitative fluorescence staining and TUNEL assays. ONOO--induced apoptosis in rat aortic SMCs appears to involve activation of Ca2+-dependent endonucleases. Although the precise mechanisms by which peroxynitrite induces apoptosis in rat aortic SMCs need to be further investigated, the present, preliminary findings could be used to suggest that ONOO- formation in the vasculature may play roles in the processes of vascular diseases, such as atherosclerosis, hypertension, and restenosis, via adverse actions on blood vessels.