The blood flow through the anterior descending branch of the left coronary artery in 4 anesthetized open-chest dogs was calculated on the basis of a new roentgen cinedensitometric technique and compared with the flow as measured simultaneously by an electromagnetic flowmeter. The densitometric flow calculation was based on the principle that the quotient of the volume of an arterial segment and the mean circulation time necessary to traverse this segment are equal to the volume flow. Injections of contrast medium were made while respiration was stopped and the heart was paced from the right atrium. There was no systematic deviation between the densitometric and the mean electromagnetic flow measurement during the passage of the contrast bolus; the correlation coefficient between simultaneous measurements was 0.932. Due to the injection of the contrast medium, an oscillation in flow occurred but small doses caused no significant change compared with the preinjection value. Coronary angiocardiography thus may be used for flow measurement through single coronary arteries in intact conscious man.