The cysteinyl leukotrienes LTC4, LTD4, and LTE4 are important mediators of human bronchial asthma. Pharmacologic studies have determined that cysteinyl leukotrienes activate at least 2 receptors, CysLTR1 and CysLTR2. Both receptors are members of the superfamily of G protein-coupled receptors. CysLTR2 seems to play a major role in endocrine and cardiovascular systems. Stimulation of CysLTR2 by BAY u9773, a partial agonist, induces specific contractions of pulmonary veins and might also have an indirect role in the relaxation of the pulmonary vascular endothelium.