Abstract. We cultured smooth muscle cells as explants from rat mesenteric arterioles (40200m in diameter) obtained by injecting a suspension of iron oxide intraarterially and magnetically separating the arterioles after collagenase digestion of adventitial tissue. In third-passaged cells we ascertained smooth muscle purity of 98% by characteristic morphology, contraction responses, and specific immunofluorescence staining. Treatment of growth-arrested (in 0.4% fetal calf serum) cells with platelet-derived growth factor (0.37.5nM) or angiotensin II (0.0011000nM) induced 3H-thymidine incorporation and cell proliferation in a dose-dependent manner (P0.01). S-nitroso-N acetylpencillamine (0.050.5mM), a nitric oxide-generating compound, inhibited 10% fetal calf serum-induced 3H-thymidine incorporation (P0.05) and cell proliferation (P0.01). The antimitogenic effect of S-nitroso-N-acetylpencillamine was significantly reduced by hemoglobin and potentiated by superoxide dismutase (P0.01). In addition to a new technique for culturing mesenteric arteriolar smooth muscle cells, these findings provide evidence that platelet-derived growth factor, angiotensin II, and nitric oxide may be involved in their growth control.