Tetrahydrobiopterin (BH 4) acts as an important co-factor for endothelial nitric oxide synthase (eNOS). Glucocorticolds have been shown to inhibit expression of the rate-limiting enzyme for tetrahydrobiopterin synthesis, GTP cyclohydrolase, in other cell types. We hypothesized that endothelium-dependent vasodilator responses would be blunted in rats made hypertensive with dexamethasone. Further, we hypothesized that treatment of rat vascular segments with dexamethasone would result in attenuation of endothelial function accompanied by decreased GTP cyclohydrolase expression. We report that endotheliumdependent relaxation responses to the calcium ionophore A23187 are reduced in aortic rings from dexamethasonehypertensive rats compared with sham values. Dexamethasone incubation abolishes contraction to N...-nltro-t-argtnlne (L-NNA, 10-5 M) in endothelium-intact aortic rings, and inhibits expression of GTP cyclohydrolase. We conclude that inhibition of BH 4 synthesis by glucocorticoid regulation of GTP cyclohydrolase expression may contribute to reduced endothellumdependent vasodilation characteristic of glucocortlcoid-induced hypertension.