The experiments were performed on open-chest mongrel dogs, the hearts being paced at a constant rate. Both vagi were cut in the neck and the peripheral ends were stimulated with frequencies of 1, 5, 10, 15, 20, 25, 30, 35, 40 and 50 cycles per sec (30 sec, 8 V, 2 msec). Adrenergic beta-receptors were blocked with propranolol and adrenergic alpha-receptors with dihydroergotamine. Coronary flow was measured with electromagnetic flowmeters on the descending branch of the left coronary artery. A significant increase in coronary flow was observed with rising frequency of stimulation. This increase was 15 percent with 30 Hz and decreased with higher frequencies. The total coronary resistance reached its lowest level at the frequency of 30 cycles per sec. The results clearly indicate the parasympathetic control of myocardial blood flow.