Basic fibroblast growth factor (FGF) is a multifunctional peptide that may play an integral role in angiogenesis associated with coronary collateral formation and myocardial infarct healing. We sought to determine the effects of exogenously administered basic FGF on collateral blood flow to ischemic myocardium. Ameroid constrictors were used to cause gradual occlusion of the left circumflex coronary artery in dogs. Basic FGF (110 micrograms, n = 9) or saline (n = 12) was given as a daily bolus injection directly into the collateral-dependent zone, beginning 10 days after placement of the Ameroid and continuing for 28 days. Collateral flow was assessed weekly as the ratio of collateral to normal zone (CZ/NZ) blood flow during maximal pharmacologically induced coronary vasodilation. The CZ/NZ increased in both treated and control dogs as a function of time; however, transmural collateral flow in basic FGF-treated dogs significantly exceeded that of control dogs by the second week of treatment. Final CZ/NZ blood flow ratios were 0.49 +/- 0.05 and 0.35 +/- 0.02 in the treated and control groups, respectively (means +/- SE, P = 0.0002). Treatment with basic FGF was also associated with significant increases in the numerical density of distribution vessels and endothelial cell DNA synthesis within the CZ. We also found that basic FGF had acute effects as a coronary vasodilator. Thus exogenous administration of basic FGF enhances maximal collateral blood flow in dogs with myocardial ischemia secondary to single-vessel coronary occlusion, an effect that is likely mediated through the direct angiogenic effects of the peptide, although its acute vasodilatory effects may also play a role.