Among the three G-protein-linked acetylcholine (ACh) receptors (GAR-1, -2, and -3) in Caenorhahditis elegans (C elegans), GAR-3 appears most similar to mammalian muscarinic ACh receptors (mAChRs). The gar-3 gene, unlike mammalian mAChR genes, contains introns within the coding region. in this study, we identified an alternatively spliced isoform of GAR-3 (GAR-3a), which differs only in the third intracellular (0) loop from the previously described one (GAR-3b). GAR-3a has a 26 amino acid insert in the D loop compared with GAR-3b. Reverse transcriptase-polymerase chain reaction (RT-PCR) on stage-specific RNAs indicated that both isoforms are expressed at all developmental stages examined, with gar-3b being more abundantly expressed than gar-3a. When these two GAR-3 isoforms were expressed in Chinese hamster ovary (CHO) cells, they exhibited similar ligand binding characteristics. In response to carbachol treatment, the two isoforms stimulated phosphatidylinositol hydrolysis with similar efficacy. Together with our earlier observations that GAR-1 and GAR-2 undergo alternative splicing, this study shows that alternative splicing plays an important role in promoting molecular diversity of G-protein-linked ACh receptors in C. elegans. (C) 2003 Elsevier Inc. All rights reserved.