The bidirectional ultrasonic Doppler method enables one to detect transcutaneously the flow patterns in blood vessels. The velocity spectrum consisting of various velocity components in the vessel can be shown by sound spectrography. 2. In healthy subjects, the arterial blood flow pattern of a single pulse cycle consisted of a rapid early forward flow which was usually followed by a small reverse phase, and a subsequent forward flow in diastole. It was noted that the conversion phase of flow direction showed a simultaneous existence of forward and reverse in the vessel. 3. The arterial flow pattern in such diseases as aortic regurgitation, aortitis syndrome and idiopathic hypertrophic subaortic stenosis showed respective features. 4. Venous flow patterns were also examined. The flow pattern in the internal jugular vein consisted of two peaks in a pulse cycle. This fundamental pattern was easily modified by certain physiological or pathological conditions.