To investigate the possible effect of heparin on coronary-collateral growth, dogs were instrumented chronically with a left circumflex (LCF) flow probe, an ameroid constrictor, a balloon occluder, and left and right atrial and aortic catheters. Collateral blood flow was measured at least weekly or when the coronary artery was occluded. Animals were allocated randomly to a control or treatment group. The latter received a continuous right-atrial infusion of a heparin solution at a rate of 2 to 3 ml/hour and a concentration adjusted daily to increase the activated clotting time to approximately 3 times the pre-drug level. After LCF occlusion, coronary-collateral flow was measured during exercise. There was no difference in time to LCF occlusion in the anticoagulated dogs. Furthermore, neither the rate of collateral development, the magnitude of collateral flow shortly after spontaneous LCF occlusion, nor the degree of collateral vascular reserve during exercise was different in the two groups. Therefore, in this model of coronary obstructive disease, heparin does not accelerate coronary-collateral development.