The non-ionic contrast medium metrizamide produced a smaller coronary flow increase than the ionic medium diatrizoate after injection into the main left coronary artery in dogs and had also a shorter transit time through the coronary vessels. The coronary veins were better filled with metrizamide. The reduced coronary flow effects of metrizamide compared to ionic contrast medium and the improved venous filling makes metrizamide a favorable contrast material for use in coronary angiography.