The flow section is a small (1-3 cm) lucite cylinder which is clamped about the blood vessel. Piezoelectric crystals are mounted on the flow section so that bursts of 3-mc sound may be transmitted alternately upstream and downstream. The flowmeter develops a voltage which is proportional to the difference in the upstream and downstream transit times of the sound. This voltage is recorded continuously and calibrated in terms of flow. Under optimal conditions, the output voltage is a linear and accurate representation of volume flow within ?±5 per cent, independent of the velocity profile. The flowmeter responds to a step variation in flow within 0.01 second. The maximum noise and baseline drift is equivalent to a flow velocity variation of less than 1 cm/second measured over a 4-hour period.