OBJECTIVE To characterise the effect of coronary intra-arterial thrombosis upon the downstream vascular bed. BACKGROUND The vascular response downstream from a coronary intra-arterial thrombus has not previously been characterised. We postulated that downstream vasoconstriction might result from the presence of endothelial damage with consequent growth of platelet-rich thrombus. METHODS We measured the pressure gradient and flow across, and the pressure/flow ratio distal to, a canine left circumflex artery stenosis with and without endothelial damage causing intracoronary thrombosis. We also observed the effects of transient complete conclusions. RESULTS At occlusion, the pressure gradient was maximal; relief of occlusion caused a rapid increase flow and distal pressure with a rapid decrease in stenosis pressure gradient and resistance. Subsequently there was a period of stable stenosis resistance with pressure gradient and flow declining; distal pressure therefore increased at this time. Finally in the thrombus group only, stenosis resistance increased again towards re-occlusion. During occlusion, distal pressure averaged 49 +/- 18 mmHg in the presence of thrombus vs. 22 +/- 4 mmHg in its absence (P < 0.001). Following release of occlusion, the flow increased faster than distal pressure, so that the ratio (distal pressure/flow) fell rapidly. Subsequently, distal pressure continued to increase after flow had reached a peak and begun to decline, suggesting vasoconstriction. In the presence of thrombus, the distal pressure/flow ratio was higher than in the absence of thrombus, both at maximal vasodilation (P < 0.005) and at maximum vasoconstriction (P < 0.025). CONCLUSIONS During cyclic flow variations the stenosis resistance changes are exactly as expected from thrombus growth and embolisation. The distal pressure/flow ratio showed a time-dependent increase which appeared greater when conditions favoured intracoronary thrombosis.