The purpose of this study was to determine whether blood flow through chronic collateral vessels may reverse its direction to supply acutely ischemic myocardium. Ameroid constrictors were placed on the circumflex coronary artery (CCA) of 12 dogs to promote collateral flow (CQ) from the left anterior descending (LAD) to the CCA. Twelve weeks later, myocardial blood flow (MBF) was determined (ml/g/min) using radioactive tracer microspheres. Control LAD subepi-cardial (EPI) and subendocardial (ENDO) flows were 1.17 ± 0.11 and 1.00 ± 0.10 (Mean ± SEM), respectively. CCA EPI and ENDO flows were 1.25 ± 0.12 and 1.12 ± 0.20, respectively. The LAD was then occluded, and MBF to the CCA bed decreased by an average of 0.31 ± 0.03 ml/g/min (P < 0.005). This decrease in MBF quantitated the amount of CQ from the LAD to the CCA bed. An aortocoronary bypass graft was anastomosed to the distal CCA and MBF to the LAD bed immediately increased by an average of 0.30 ± 0.02 ml/g/min (P < 0.005). This increase in MBF represented reversed CQ from the CCA to the acutely ischemic LAD bed. It had a normal transmural distribution (ENDO/EPI = 0.9). Four hours later, total reversed CQ to the LAD bed remained unchanged, but was redistributed toward the EPI (ENDO/EPI = 0.6). These data document that chronic collateral vessels are capable of immediate and sustained conduction of CQ in a retrograde direction and suggest that these collateral vessels may play a role in limiting the degree and transmural extent of ischemic injury when a perioperative myocardial infarction occurs in the vascular bed of a nonbypassed coronary artery.