The cysteinyl leukotrienes, leukotriene C4 (LTC4), leukotriene D4 (LTD4) and leukotriene E4 (LTE4), activate contractile and inflammatory processes via specific interaction with putative seven transmembrane-spanning receptors that couple to G proteins and subsequent intracellular signaling pathways. Pharmacological characterizations identified at least two subtypes of cysteinyl leukotriene (CysLT) receptor based on agonist and antagonist potency for biological responses. The rank potency of agonist activation for the CysLT1 receptor is LTD4 > LTC4 > LTE4 and for the CysLT2 receptor is LTC4 = LTD4 > LTE4. CysLT1 selective receptor antagonists are efficacious in the treatment of asthma. No selective CysLT2 receptor antagonists have been described. Molecular identification of the human and mouse CysLT1 and CysLT2 receptors has confirmed their structure as putative seven transmembrane domain G protein-coupled receptors and largely confirmed the previous pharmacological characterizations. The CysLT1 receptor is most highly expressed in spleen, peripheral blood leukocytes including eosinophils, and lung smooth muscle cells and interstitial lung macrophages. The CysLT2 receptor is most highly expressed in the heart, adrenal medulla, placenta and peripheral blood leukocytes. The molecular identification of the mouse CysLT1 and CysLT2 receptors show similar but not identical profiles to the orthologous human receptors.