Velocity profile of the portal blood flow in dogs was measured intraoperatively using an 8 MHz pulsed Doppler flowmeter. Axial size of the sample volume was 0.33 mm. This sample volume was repositioned automatically in 0.33 mm increments using an ECG-R wave trigger. The velocity profiles obtained were approximated to the equation, V(r)=Vmax[1-(r/a)n], where r is the distance from the axis, a is the diameter of the vessel, and Vmax is the maximum velocity along the axis. Then the profile index n and Vmean/Vmax ratio were calculated from this equation, where Vmean is the mean velocity of the obtained profile. The profiles were measured both in the portal vein trunk and the superior mesenteric vein at various flow rates, these being varied using stenosis and continuous injection of PGE1 to the superior mesenteric artery. The average value of ʻʻnʼʼ was 2.33±0.27 (mean±S.D., N=24) and the calculated Vmean/Vmax ratio was 0.54±0.03. As the flow rate increased, the velocity profile became increasingly parabolic.