Myocardial blood flow responses to selective coronary arteriography were studied in 22 patients. Left coronary flow was estimated by the radioactive inert-gas (133xenon) washout technic using precordial detection. Duplicate control studies were obtained before radiocontrast injection and test studies were performed at 1 min and 3, 5, or 7 min after angiography. Eleven patients received a single 7-ml injection of methylglucamine diatrizoate and 11 were given a series of injections (average total 40 ml), with the test blood flow studies after the final injection.Myocardial flow increased in every patient 1 min after contrast injection (average control flow 66.3 ml/min/100 g; test flow 78.8, +18.9%,P< 0.001). Blood flow increased further in seven of 10 studied at 3 min (average 95.7 ml/min/100 g, +35.7%,P< 0.001) and remained elevated at 5 and 7 min. Systemic pressure was unchanged from the control level, so coronary vascular resistance decreased in proportion to the increase in flow. Blood flow responses in patients with coronary artery disease were no different from normal subjects. Sham injections of normal saline produced no increase in coronary flow.Selective coronary arteriography results in a prompt increase in myocardial blood flow which persists for several minutes. The probable mechanism is coronary vasodilatation produced by the markedly hypertonic contrast material, an effect demonstrated previously in other vascular beds.