A conscious, chronically instrumented canine model was used to investigate resistance changes in the distribution of the circumflex coronary artery as the artery was constricted. Several discrete constrictions were studied at two different levels of flow: resting and peak flow reactive hyperaemia. The resistance of the bed downstream from the constriction was calculated by formulae which defined the coronary back pressure as either venous pressure (Pv) or the arterial pressure at which coronary flow ceased (Pc). When Pv was taken as the back pressure, the resistance of the coronary bed during reactive hyperaemia progressively increased as the upstream artery was constricted. When Pc was taken as the back pressure, calculated coronary resistance for reactive hyperaemia showed little change. It is not known if elastic recoil of coronary resistance vessels, consequent to low poststenotic pressure, could occur to the extent required to be the physical basis for the calculated increase in resistance when Pv is taken to be the back pressure. Use of Pc as the back pressure implies that the coronary circulation contains a segment having the hydraulic characteristics of a collapsible tube.