The paradoxical effects of cysteinyl-leukotrienes, namely contraction and relaxation, are now well documented in a number of vascular preparations from various species. The vascular smooth muscle contractions are associated with activation of a single receptor subtype and in some vascular smooth muscles with activation of two receptor subtypes. However, the receptors implicated in the contraction of vessels such as pig pulmonary arteries and veins, dog inferior vena cava, and dog splenic and mesenteric veins remain to be established. There are sufficient data concerning some vascular tissues to suggest that relaxations induced by cysteinyl-leukotrienes are via the stimulation of specific receptors present on the endothelium. The endothelium in human pulmonary arteries has one receptor (CysLT2) and activation induced the release of NO. However, in isolated human pulmonary veins two receptors are present, CysLT1 and CysLT2 (Figure 1). Activation of the former induced the release of a contractile factor whereas activation of the CysLT2 receptor released NO. In guinea pig pulmonary artery and guinea pig thoracic aorta, one receptor has been demonstrated since the relaxations are blocked by ICI-198615. These data suggest the presence of a CysLT1 receptor. Activation of this receptor leads to the release of a relaxant factor, namely, nitric oxide. In contrast, in human pulmonary arteries and veins activation of a receptor that is resistant to ICI-198615 is associated with NO release. These results suggest that there may be species differences even when analogous vascular preparations are examined. While the cysteinyl-leukotrienes are known to relax vascular smooth muscle in a variety of preparations from different species, there are presently two pathways known to be involved in this response. One involves the metabolites of arachidonic acid via the cyclooxygenase enzymatic pathway and the other implicates products of the L-arginine enzymatic pathway. Although both pathways may be present and active in the endothelium of the vascular preparations only one of these enzymatic pathways may be dominant and responsible for the relaxations observed. Ortiz and coworkers have demonstrated that in pulmonary veins the dominant pathway for cysteinyl-leukotriene relaxations is the NO pathway. There are some reports from animal studies that support a dominant role for NO in pulmonary veins. In contrast, Allen and co-workers demonstrated that the LTC4-induced relaxations in isolated human saphenous veins were not modified by treatment of tissues with an NO inhibitor but were significantly enhanced after treatment with indomethacin. These authors suggested that a contracting factor derived from the arachidonic acid pathway was released in preparations challenged with LTC4. In addition, these investigators demonstrated that the NO inhibitor had no effect on the LTC4 relaxations. Together, these results suggest that cysteinyl-leukotriene effects in human pulmonary veins are dominated by the NO pathway whereas in human systemic veins these mediator effects are modified by metabolites of the cyclooxygenase pathway. Unfortunately, most studies involving the actions of cysteinyl-leukotrienes on vessels have been performed in the presence of indomethacin, making interpretation of the relative contribution of the cyclooxygenase and NO pathways difficult. In any event, the cysteinyl-leukotrienes may have a prominent role in the activation of these pathways and the receptors involved have not been clearly established.