OBJECTIVE The aim was to investigate the bidirectional functional adequacy of collateral perfusion in conscious dogs. METHODS Left circumflex coronary artery (LCCA) occlusions of 1 or 2 min duration were repeated to stimulate the development of collateral perfusion to the LCCA area, and the left anterior descending coronary artery (LAD) was occluded once daily to evaluate the development of retrograde LCCA-LAD flow. METHODS 7 male mongrel dogs were used, weight 25-28 kg. RESULTS Coronary collateral flow from the LCCA to the LAD perfusion area was measured as the abrupt decrease in the LCCA flow (implanted Doppler transducer) upon release of a brief LAD occlusion. Measurements were repeated daily during the development of collaterals induced by repeated, brief occlusions of the LCCA. After 35(SD17) days of such occlusions; there was no sustained reduction in LCCA regional myocardial function during an LCCA occlusion, and reactive hyperaemic repayment following the occlusion was negligible. Before and after collateral development, the LCCA to LAD collateral flow increased from 1.1(0.2) to 8.6(5.1) cm.s-1. LAD systolic segment shortening during the LAD occlusion increased from 2.1(2.0)% (first occlusion) to 19.3(8.6)% (last occlusion). CONCLUSIONS LAD to LCCA collaterals serve as functionally significant bidirectional perfusion conduits, and monitoring of collateral perfusion development is practical by measuring the step reduction in LCCA flow upon abrupt release of an LAD occlusion.