The objective of this study was to determine whether intramural collaterals contribute significantly to total retrograde flow (index of collateral flow). The left circumflex, left anterior descending, right, and septal coronary arteries were separately cannulated, and blood flows through these vessels were monitored on an isolated, blood-perfused beating heart preparation. Epicardial collaterals between the borders of the circumflex and right coronary perfusion territories were cauterized, and retrograde flow from the circumflex coronary artery was determined before and after cauterization. This procedure unmasked the intramural collateral flow component to retrograde flow. By occluding and unoccluding the coronary flows from the right, septal, and left anterior descending coronary arteries during these measurements, we were able to determine their contribution to epicardial and intramural collateral flow. We found that, after cauterization, an average of 58 +/- 3.6% of the total retrograde flow remained. The septal and left anterior descending coronary arteries contributed almost equally to this retrograde flow. We concluded that, because the epicardial collaterals were cauterized, the source of retrograde flow was from intramural collaterals and constituted about one-half of the measured retrograde flow in dogs with a native collateral circulation.