Experiments were designed to determine the effects of chronic estrogen treatment on endothelium-dependent responses to arachidonic acid in the aorta of the rabbit. Ovariectomized rabbits were treated with either placebo or 17 beta-estradiol for 14 days. The aortas were removed, and rings cut from each blood vessel were suspended for measurement of isometric force. In rings contracted with a submaximal concentration of norepinephrine, endothelium-dependent contractions to arachidonic acid were augmented by estrogen treatment. Prostaglandin F2 alpha, E2, the thromboxane analogue U 46619, and prostacyclin contracted aortic smooth muscle. Only the contractions to prostacyclin were augmented by estrogen treatment. Estrogen treatment enhanced the contractions to norepinephrine in aortas with, but not in those without, endothelium; these contractions were not affected by inhibition of uptake of the neurotransmitter. Indomethacin inhibited contractions to norepinephrine in rings with endothelium from estrogen-treated rabbits. These results indicate that chronic treatment with estrogens can affect arterial reactivity to norepinephrine through an endothelium-dependent mechanism that may involve the metabolism of arachidonic acid by cyclooxygenase. The altered sensitivity of the smooth muscle to prostacyclin may contribute in part to the augmented contractions to arachidonic acid with estrogen treatment.