Experiments were designed to determine whether normal fluctuations in endogenous sex steroid hormones and/or gender affect endothelium-dependent relaxations in coronary arteries, and, if so, to identify endothelium-derived factors contributing to these differences. Coronary arteries from sexually mature, gonadally intact male and female pigs or ovariectomized pigs were prepared either for study of isometric force in organ chambers or for measurement of prostanoids and activity of nitric oxide (NO) synthase. In organ chamber studies, neither the magnitude nor the sensitivity of endothelium-dependent relaxations correlated with endogenous estrogen or progesterone in female pigs. However, relaxations to bradykinin and UK-14304 were significantly greater and/or shifted leftward in arterial rings from female compared with male pigs. Indomethacin (10(-5) mol/l) increased endothelium-dependent relaxations only in arteries from male pigs. N(G)-monomethyl-L-arginine reduced endothelium-dependent relaxations to a similar extent in coronary arteries from either sex. Neither production nor response to thromboxane A2 or prostacyclin differed in coronary arteries from male compared with female pigs. Activity for calcium-dependent or -independent NO synthase was similar in both sexes. These results suggest that normal fluctuations in endogenous sex steroid hormones do not affect endothelium-dependent relaxations in coronary arteries from female pigs. There are, however, gender differences in endothelium-dependent relaxations that are indomethacin sensitive and may be due to cyclooxygenase products other than thromboxane A2 or prostacyclin.