Two classes of human G protein-coupled receptors, cysteinyl leukotriene 1 (CysLT₁) and CysLT₂ receptors, recently have been characterized and cloned. Because the CysLT₁ receptor blockers are effective in treating human bronchial asthma and the mouse is often used to model human diseases, we isolated the mouse CysLT₁ receptor from a mouse lung cDNA library and found two isoforms. A short isoform cDNA containing two exons encodes a polypeptide of 339 aa with 87.3% amino acid identity to the human CysLT₁ receptor. A long isoform has two additional exons and an in-frame upstream start codon resulting in a 13-aa extension at the N terminus. Northern blot analysis revealed that the mouse CysLT₁ receptor mRNA is expressed in lung and skin; and reverse transcription–PCR showed wide expression of the long isoform with the strongest presence in lung and skin. The gene for the mouse CysLT₁ receptor was mapped to band XD. Leukotriene (LT) D₄ induced intracellular calcium mobilization in Chinese hamster ovary cells stably expressing either isoform of the mouse CysLT₁ receptor cDNA. This agonist effect of LTD₄ was fully inhibited by the CysLT₁ receptor antagonist, MK-571. Microsomal membranes from each transformant showed a single class of binding sites for [³H]LTD₄; and the binding was blocked by unlabeled LTs, with the rank order of affinities being LTD₄ >> LTE₄ = LTC₄ >> LTB₄. Thus, the dominant mouse isoform with the N-terminal amino acid extension encoded by an additional exon has the same ligand response profile as the spliced form and the human receptor.