Objective: To establish the influence of fetal behavior states on venous and arterial pulmonary blood flow velocity waveforms in the normally developing term fetus. Methods: The relation between venous and arterial pulmonary blood flow velocity waveforms and fetal behavior states was investigated in 18 normal term fetuses. Recordings of the venous pulmonary blood flow velocity waveforms were obtained just proximal to the entrance in the left atrium, and the arterial pulmonary blood flow velocity waveforms were taken from the most proximal branch of the pulmonary artery in the same lung using color Doppler imaging. Time-averaged peak systolic, peak diastolic, and end-diastolic flow velocity; peak systolic to peak diastolic ratio; pulsatility index; and fetal heart rate were calculated from both venous and arterial Doppler recordings obtained during behavior states 1F (quiet sleep) and 2F (active sleep). Fetal behavior states were determined from combined recordings of fetal eye and body movements. Results: Recordings of sufficient quality for analysis were obtained from ten fetuses. Venous pulmonary blood flow velocity waveforms demonstrated a statistically significant increase in time-averaged peak diastolic and end-diastolic velocity during fetal behavior state 2F. No behavior-state-related changes were observed for the arterial pulmonary blood flow velocity waveform. Conclusion: The data suggest an increased pressure gradient between the pulmonary venous system and the left atrium during behavior state 2F. Flow velocity waveforms from the proximal arterial pulmonary branch are independent of behavioral state.