Coronary flow per minute per 100 Gm. of heart with venous drainage into the coronary sinus as measured by the nitrous oxide procedure was compared in the dog with left coronary artery inflow per 100 Gm. of heart perfused by this artery as measured by the rotameter. The error arising from epicardial leak of nitrous oxide was prevented or quantitated by appropriate coverage of the heart. Extremes of the comparisons were +21 and -22 per cent, with an average variation of ±12.4 per cent. The nitrous oxide method is regarded as giving semiquantitative coronary flow values per 100 Gm. of left heart in the dog.