Smooth muscle cell proliferation occurs in the pulmonary artery throughout embryonic and fetal life, but the factors controlling their growth remain unknown. It is likely that growth factors responsible for mitogenesis of the pulmonary artery smooth muscle cell may also play a role in the smooth muscle cell proliferation observed in vascular injury or in unregulated smooth muscle cell proliferation. In this study, aortic and pulmonary smooth muscle cells were cultured as explants from the 16 gestation day rat and in vitro growth was characterized. The smooth muscle cells in utero exhibited a “hill & valley” morphology typical of adult smooth muscle cells in vitro and uniformly stained with a smooth muscle-specific alpha actin antibody. Both embryonic smooth muscle cell types (aortic & pulmonary) displayed autocrine growth in culture. The nuclear replication rate for a 24-hr period was 30 - 40% when the smooth muscle cells were grown in serum-free media. In addition, the cells continued to divide in serum-free culture media for up to eight days without a change in medium. When bovine pituitary extract was added to the serum-deprived medium, the pulmonary artery smooth muscle cells showed increased nuclear labeling up to 60% with 50 mcg/ml of bovine pituitary extract. This is contrasted with the aortic smooth muscle cells in which no additional labeling occurred in the presence of bovine pituitary extract treatment. The pulmonary smooth muscle cells also demonstrated a mitogenic response when they were treated in vitro with basic FGF but not with acidic FGF. The aortic smooth muscle cells showed no response to either factor. The data describe a phenotype of embryonic pulmonary artery smooth muscle cells that is self-driven, like the aortic smooth muscle cells, but that can be mitogenically induced with basic FGF. This difference in mitogenic potential between embryonic aortic and pulmonary smooth muscle cells may be an important factor distinguishing vascular morphogenesis of the two vessels.