Pulsed wave Doppler ultrasound recordings of blood flow velocity in the superior vena cava were made in 40 healthy adults (aged 22 to 69 years) during both normal respiration and 10 second episodes of apnea. The forward flow velocity pattern was biphasic, with systolic flow velocity greater than diastolic flow velocity. During apnea, peak flow velocities ranged from 32 to 69 cm/s (mean 45.7 +/- 8.4) during systole and from 6 to 45 cm/s (mean 27.2 +/- 8.3) in early diastole. Systolic flow velocity integrals also exceeded diastolic values. With atrial systole (A wave), forward flow velocities were reduced or flow was reversed. Thirty-nine of 40 subjects had A wave flow reversal during apnea, and in these the ratio of reverse to total forward flow velocity integrals ranged from 1 to 16% (mean 6 +/- 4%). Compared with values during apnea, there were higher mean values with inspiration and lower values with expiration for velocities and flow velocity integrals. Hepatic vein tracings, when adequate (12 of 40 subjects), showed forward flow characteristics similar to those from the superior vena cava, but with more frequent and larger A wave and ventricular end-systole (atrial V wave) flow reversals. Superior vena cava flow velocity variables were calculated in subgroups to assess the effects of age, respiratory pattern and increased venous return. This study defines normal Doppler ultrasound superior vena cava and hepatic vein flow velocities and their variation with respiration in healthy adults. These results can be used for comparison with patterns found in disease states.