Induction of NO synthase expression by interleukin-1 beta in cultured vascular smooth muscle cells from rat aortas was accompanied by simultaneous induction of GTP: cyclohydrolase I. This enzyme regulates the de novo synthesis pathway for tetrahydrobiopterin, an essential cofactor for the catalytic conversion of L-arginine to L-citrulline and NO by inducible NO synthase. Inhibition of GTP: cyclohydrolase attenuated NO production by interleukin-1 beta-stimulated smooth muscle cells. Peptide growth factors such as fibroblast growth factor, platelet-derived growth factor and transforming growth factor beta 1 and the protease thrombin have been shown to modulate the production NO by cytokine-treated smooth muscle cells. These peptide agonists also regulated the induction of NO synthase and GTP: cyclohydrolase mRNA expression.