Three different isoforms of the inhibitory guanine nucleotide-binding protein α-subunit (GIα) have been cloned. However, little information on the cardiac-specific expression of these isoforms is available. The deduced amino acid sequence of a full-length GIα2 complementary DNA from a canine ventricular library displays several unique residues compared with those from other species. In contrast to previously described complementary DNA clones for GIα2 this complementary DNA has a complete 3'-untranslated region with the expected consensus sequence for polyadenylation. GIα2 is the predominant isoform in the heart, and its message level of 14.7 ± 4.9 pg/10 μg total ventricular RNA is about fourfold lower than that of the stimulatory guanine nucleotide-binding protein.