To understand hepatic vein (HV) and portal vein (PV) duplex waveforms and their normal and respiratory variations, HV and PV duplex sonography with simultaneous electrocardiography was performed on 11 volunteers. Absolute velocities of the waveforms' components and their ratios were determined at mid-inspiration, full inspiration, full expiration, and Valsalva maneuver. The normal HV waveform was variable in shape and component velocities and ratios but essentially consisted sequentially of (1) an antegrade systolic wave resulting from movement of the tricuspid annulus toward the cardiac apex and occurring shortly after QRS; (2) a retrograde v-wave resulting from atrial overfilling and occurring immediately after the T-wave; (3) an antegrade diastolic wave resulting from opening of the tricuspid valve and occurring shortly after the T-wave; and (4) a retrograde a-wave resulting from atrial contraction and occurring immediately after the P-wave. The ratio of the maximum systolic velocity to maximum diastolic velocity varied from 1.0 to 2.8 (mean 1.4). Systolic-to-diastolic ratio decreased during inspiration but was always greater than 0.6 and increased during expiration. The Valsalva maneuver diminished waveform pulsatility. PV waveforms were more triphasic than biphasic but less pulsatile, flow was totally antegrade, and respiratory changes were less remarkable than HV waveforms. All normal HV and most normal PV waveforms showed multiphasicity that corresponded to cyclic cardiac changes. The shapes of these waveforms were variable and were modified by respiratory movements.