Previous studies reported sex differences in production of endothelium-derived substances and suggested that these compounds may be involved in regulation of vascular tone under both normal and pathological conditions. The present study was designed to compare the effects of endothelium-dependent contractions in pulmonary artery vessels obtained from male and female rabbits. Rings of intrapulmonary arteries were suspended under isometric tension in oxygenated Krebs' buffer. In male rabbit pulmonary artery, arachidonic acid and methacholine elicited endothelium-dependent, concentration-related contractions (maximal contraction, 79 +/- 4% and 54 +/- 4% of the KCl contractions, respectively). In contrast, endothelium-dependent arachidonic acid- and methacholine-induced contractions were greater in female pulmonary arteries (maximal response, 113 +/- 7% and 101 +/- 6% of the KCl contractions, respectively). There was no difference in KCl-induced contractions in female and male pulmonary arteries (1.2 +/- 0.1 versus 1.3 +/- 0.1 g, respectively). In male rabbits, the vasoconstrictor responses to arachidonic acid and methacholine were inhibited by the cyclooxygenase inhibitor indomethacin. We have previously identified thromboxane A2 as the endothelium-dependent contracting factor in male rabbits. However, indomethacin only partially inhibited arachidonic acid-induced contractions in female pulmonary arteries (maximal inhibition, 46% of the control response) suggesting that a noncyclooxygenase metabolite of arachidonic acid mediates contraction in female rabbits. Likewise, indomethacin only partially inhibited methacholine-induced contractions of female pulmonary arteries. The combined cyclooxygenase/lipoxygenase inhibitor BW 755C and the lipoxygenase inhibitor nordihydroguaiaretic acid completely blocked arachidonic acid-induced contractions in females. Furthermore, both basal and stimulated production of thromboxane B2, as measured by radioimmunoassay, were similar in female and male pulmonary arteries. When segments of pulmonary arteries obtained from female and male rabbits were incubated with 14C-arachidonic acid and the extracted metabolites were resolved by reverse-phase high-performance liquid chromatography, there was an enhanced production of metabolites in females. Pretreatment with indomethacin attenuated metabolism of all products in the males but enhanced production of some compounds in vessels from the females. These observations suggest that the enhanced vasoconstrictor response to arachidonic acid in female pulmonary arteries is due to a lipoxygenase metabolite of arachidonic acid.