In the rabbit, 5,6-epoxyeicosatrienoic acid (EET) was reported both to dilate and constrict pulmonary blood vessels. We propose that these seemingly contradictory results could be explained by differences in responses to 5,6-EET in large conductance pulmonary arteries (PA) compared to smaller pulmonary arteries and resistance vessels. Thus, we found that rings of extralobar PA (> 2 mm) outside diameter (o.d.), in which active tension had been increased with prostaglandin F₂, 5,6-EET produced relaxation in a concentration- and cyclooxygenase (COX)-dependent manner. In contrast, 5,6-EET increased tension in intralobar (1-2 mm o.d.) PA. Small extralobar PA (2-2.5 mm o.d.) exhibited intermediate responses. In the intact lung, the net effect of 5,6-EET (1 x 10^-8 - 1 x 10^-5 M) was an increase in pulmonary vascular resistance (PVR) from 13.0 ± 0.5 to 47.8 ± 4.6 mm Hg ^. 100 ml^{-1 .} min^-1 (EC₅₀ 5.9 ± 1.7 x 10^-7 M). The increase in PVR was accompanied by a 10-fold increase in perfusate thromboxane (TX) B₂ concentration. The 5,6-EET-induced increase in PVR was prevented with indomethacin (100 µM), a cyclooxygenase inhibitor or ONO-3708 (20 µM), a TX/PGH₂ (TP) receptor antagonist, but not with OKY-046 (700 µM), a thromboxane synthase inhibitor. These results demonstrate that while 5,6-EET dilates large, extralobar pulmonary arterial segments in a COX-dependent manner, in the intact rabbit lung 5,6-EET produces constriction which requires synthesis of a COX-dependent agonist of the TP receptor, other than thromboxane.