Two classes of human G protein-coupled receptors, cysteinyl leukotriene 1 (CysLT(1)) and CysLT(2) receptors, recently have been characterized and cloned. Because the CysLT(1) receptor blockers are effective in treating human bronchial asthma and the mouse is often used to model human diseases, we isolated the mouse CysLT(1) 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 CysLT(1) 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 CysLT(1) 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 CysLT(1) receptor was mapped to band XD. Leukotriene (LT) D(4) induced intracellular calcium mobilization in Chinese hamster ovary cells stably expressing either isoform of the mouse CysLT(1) receptor cDNA. This agonist effect of LTD(4) was fully inhibited by the CysLT(1) receptor antagonist, MK-571. Microsomal membranes from each transformant showed a single class of binding sites for [(3)H]LTD(4); and the binding was blocked by unlabeled LTs, with the rank order of affinities being LTD(4) >> LTE(4) = LTC(4) >> LTB(4). 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.