Polymerase chain reaction amplification of a cDNA derived from dog left ventricular myocardium, using primers specific for rat insulin-like growth factor I (IGFI), exons 3 and 6, yielded the dog clone, IGFI5.1. This clone includes the signal peptide sequence, the entire coding sequence for mature dog IGFI and the C-terminal extension sequence. By analogy with the organization of the rat and human IGFI genes which encode two extension peptides, we have termed this cDNA, dog IGFIa. The deduced amino acid sequence of mature dog IGFI is identical to that of human IGFI.