.05.). Vasodilator responses to the endothelium-independent agent sodium nitroprusside (1.6 micro gram/min) were also unchanged after estrogen supplementation. However, estrogen enhanced vasoconstrictor responses to the nitric oxide synthase inhibitor NG-monomethyl-L-arginine at doses of 1, 2, and 4 micro mol/min (fall in forearm blood flow before estrogen: 13 plus/minus 9%, 20 plus/minus 7%, and 26 plus/minus 8%, respectively; after estrogen: 18 plus/minus 9%, 36 plus/minus 7%, and 47 plus/minus 7%, P = .04). Responses to vasoactive agents were unchanged after administration of placebo. Thus, in perimenopausal women, estrogen supplementation reduces blood pressure and enhances basal but not acetylcholine-induced nitric oxide release in forearm resistance arteries. (Hypertension. 1996;28:330-334.)