It has been assumed previously that the act of selectively injecting fluid through a catheter into an artery does not significantly alter hemodynamics in that vessel, provided open communication is maintained between the distal portion of the artery and the aortic lumen. In this study, blood flow was measured with an electromagnetic flow probe in distal branches of the left coronary or superior mesenteric arteries of 9 dogs while heparinized arterial blood was injected selectively into the parent vessels at varying rates up to 7 ml/sec. This resulted, in all experiments, in an abrupt and substantial rise in blood flow which was maintained for the duration of the injection. This phenomeon is due primarily to an increase in intraarterial pressure distal to the catheter tip, caused by the injection.