With current interest in digital and densitometric angiographic techniques for evaluating coronary blood flow (CBF), the immediate effects of intracoronary contrast injections on CBF are important to establish. We investigated the effects of ECG-gated intracoronary injection of contrast on CBF with different injection rates (1-6 cc/sec), volumes (1/2-6 cc), and contrast concentrations (1/2, full strength). Sixteen open-chest dogs were instrumented with electromagnetic flow probes on the left anterior descending artery. During intracoronary injections of contrast, a characteristic decrease in coronary blood flow to approximately 45% of preinjection levels was seen an average of 1.9 seconds after intracoronary contrast injection. The timing of the depression in coronary flow was most influenced by the rate of contrast injection, and the magnitude of depression in flow was influenced by both the volume and concentration of the contrast injection. Angiographic techniques using the first 6 seconds after injection to obtain CBF measurements should take these characteristic changes into account to avoid possible errors.