Adequate pulse Doppler echocardiographic (echo) studies of the fetus were performed in 15 of 16 subjects in the third trimester of pregnancy (28 to 36 weeks) using a commercially available Duplex system with a fast Fourier transform analyzer. A preliminary real-time 2-dimensional (2-D) echo examination was first performed to identify the fetal cardiac structures and obtain standard planes. The Doppler beam was then positioned along the long axis of the main pulmonary artery (PA), imaged in the standard short-axis or left ventricular-pulmonic plane, to obtain maximal Doppler frequency shifts. In 6 of 15 fetuses (40%) in whom the inner PA width could be measured, right ventricular (RV) stroke volume ranged from 1.01 to 3.76 ml. RV cardiac output ranged from 148 to 451 ml/min (128 to 239 ml/min/kg when normalized for estimated fetal weight). In 1 of 9 fetuses in whom PA width could not be reliably measured, a marked decrease (60%) in the PA flow velocity curve area and, therefore, in the stroke volume was noted during ectopic beats. In keeping with known fetal physiology, greater Doppler frequency shifts were noted in the right atrium than in the left atrium. Also, increased Doppler frequency shifts were noted in the left atrium during the open position of the valve of foramen ovale compared to its closed position. Our study shows the feasibility of studying normal and abnormal fetal hemodynamics noninvasively using Doppler echocardiography. © 1984.