Abrupt reductions in fluid shear stress induce subendothelial smooth muscle cells (SMCs) to proliferate in experimental prosthetic grafts. Platelet-derived growth factor (PDGF), an important SMC mitogen, is expressed by cultured endothelial cells and modulated by shear stress. We hypothesized that this growth factor would be modulated by changes in shear stress in vivo. Bilateral aortoiliac prosthetic grafts were implanted into five baboons. High flow was generated by construction of femoral arteriovenous fistulas on both sides. Two months later, one of the fistulas was ligated, reducing shear stress in the upstream graft by 78 +/- 6%. Four days after fistula ligation, all grafts were removed and analyzed. As previously reported, SMC proliferation in low-flow grafts exceeded that in high-flow grafts, although the neointimal area was similar. mRNA levels for PDGF-A were significantly increased in low-flow grafts compared with high-flow grafts. In situ hybridization and immunohistochemical studies localized the increased PDGF-A mRNA and protein to the luminal endothelium and subjacent SMCs. Abrupt reductions in blood flow and fluid shear stress may induce accelerated neointimal thickening by a PDGF-A-mediated mechanism, since endothelial expression of this gene is temporally and anatomically associated with neointimal SMC proliferation.