The ultrasonic duplex system, which is composed of a pulsed Doppler flowmeter and a linear array B-mode scanner, was developed for the purpose of quantitative transcutaneous measurement of blood flow. It was applied here to the study of portal blood flow in a normal series with healthy volunteers as subjects. A preliminary experiment, prepared by setting up a simulation model for blood circulation, demonstrated an excellent correlation between values obtained with the electromagnetic flowmeter (EMF) and the combined ultrasonic B-mode and Doppler system (duplex system). This was confirmed in vivo in a study of blood flow in the canine vena cava. The portal blood flow in 88 healthy adult volunteers ranging in age from 18 to 55 years (average 34.3) was studied using this equipment. Blood flow in fasting subjects who were resting was 889 +/- 284 ml/min (mean +/- SD) or 16.3 +/- 5.0 ml/min/kg body weight (mean +/- SD).