The objective of this study was to determine if methyltestosteone attenuates the effects of estrogens on the vascular reactivity of mildly atherosclerotic coronary arteries in surgically menopausal female monkeys. Impaired vasodilation is an early indicator of endothelial dysfunction, and the administration of estrogen improves the vasodilator response. Thirty-six adult female cynomolgus monkeys were ovariectomi/ed and fed an atherogenic diet for 16 weeks. During this time they received oral treatment with either (a) placebo, (b) esterified estrogens, or (c) esterified estrogens + methyltestosterone. Quantitative coronary angiography was used to measure the percent change in diameter of the left circumflex coronary artery in response to intracoronary infusions of control solution (5% dextrose) and acetylcholine (10-6 mol/liter). Coronary arteries of control (placebo-treated) animals constricted after acetylcholine infusion [-4 * 1.5% (mean ± SEM)]. Treatment with esterified estrogens alone or with methyltestosterone resulted in vasodilation (+4.4 ± 1.8% and +4.8 ± 1.8%, respectively) (p < 0.05 vs. control). These results indicate that treatment with esterified estrogens improves endotheliummediated vasodilation of atherosclerotic coronary arteries, and that the addition of methyliestosterone does not alter this response.