Vascular endothelial growth factor (VEGF), an endothelial cell-specific mitogen, has been shown to promote therapeutic angiogenesis in animal models of critical limb ischemia. Ischemic skeletal muscle is advantageous for taking up and expressing foreign genes transferred as naked plasmid DNA. Accordingly, we investigated the hypothesis that intramuscular administration of naked plasmid DNA encoding the 121-amino acid isoform of VEGF could augment collateral development and tissue perfusion in a dog hindlimb ischemia model. Unilateral hindlimb ischemia was surgically induced in Beagle dogs. Ten days later, animals received intramuscular injections of pVEGF₁₂₁ plasmid directly in the ischemic muscles. Angiogenic effects were evaluated by angiography, calf blood pressure ratio and vasomotor reserve analyses. Thirty days after gene transfer, angiographically recognizable collateral vessels were increased in pVEGF₁₂₁-treated animals compared with controls. Improvement in perfusion to the ischemic limb was documented by a significantly higher calf blood pressure ratio for pVEGF₁₂₁ (0.79 ± 0.05) versus controls (0.56 ± 0.14, P