Using pulsed Doppler ultrasound combined with a real-time B-mode image, changes in the central venous blood flow and tracheal fluid flow were recognized during fetal breathing movements. The blood flow in the umbilical vein was increased with fetal inspiration in which the abdominal wall of fetus moved inward, and decreased with expiration in which the abdominal wall moved outward. Velocity in the fetal vena cava revealed a complex blood flow, in accordance with both fetal cardiac motions and breathing movements. The tracheal flow velocity during fetal breathing was measured at a maximum of 40 cm s-1 and the flow volume was estimated at a maximum of 6 ml breath-1 in the matured fetus.