Insulin-like growth factor-I (IGF-I) plays a fundamental role in postnatal mammalian growth, development, and metabolism. The mammalian IGF-I gene contains at least six exons from which several different mRNAs are transcribed. In this study, IGF-I mRNA in Songliao black pig liver was investigated using RLM-RACE. Using a 3'-RACE technique, we determined that all mRNA transcripts lacked exon 5 sequence and contained only exon 6 sequence. Using a 5'-RACE technique, we investigated the presence of class 1 and class 2 IGF-I mRNAs. In several other species, the class 1 and class 2 IGF-I polypeptides are generated from mRNAs containing exon 1 or exon 2, respectively. Both class 1 and class 2 IGF-I mRNAs were identified in Songliao black pig liver. Transcription is initiatated upstream of exons 1 and 2 at multiple dispersed start sites to yield two distinct IGF-I mRNA transcript classes which differ in the precursor peptides predicted from their individual leader sequences. Tissue distribution of Songliao black pig class 1 and class 2 IGF-I mRNA was investigated by real-time RT-PCR. Both classes of IGF-I mRNA were expressed in a variety of tissues, however, class 1 IGF-I mRNA was more abundant than class 2 in all tissues.