Platelet-derived growth factor (PDGF) is a potent stimulator of vascular smooth muscle cell growth. Two isoforms of PDGF A-chain mRNA that either include (long form) or exclude (short form) exon 6 are produced as a result of alternative splicing in mouse, rabbit, and human. The short form of PDGF A-chain is expressed in both resting and activated cells, while the long form is present predominantly in activated cells. Reverse transcription-polymerase chain reaction analysis with primers encompassing exon 6 revealed the presence of both long- and short-form PDGF A-chain transcripts in rat vascular smooth muscle cells. The nucleotide sequences of exon 6 and its intron boundaries were determined from rat vascular smooth muscle cell cDNA and rat leukocyte genomic DNA. Translation of the long form of PDGF A-chain mRNA was shown to terminate in the 70-base pair exon 6. Conserved sequences that may contribute to the regulation of alternative RNA splicing were identified in intron 5.