The supine position in late pregnancy is associated with many hemodynamic changes caused by compression of the inferior vena cava and a resulting fall in cardiac output. To investigate the effect of this position on uteroplacental blood flow, 22 women with normal (N = 12) or hypertensive (N = 10) pregnancy were investigated using the intravenous133Xe washout method, in both the supine and left-tilted (45-degree) lateral positions. The intervillous blood flow (113 ± 48 ml/min/dl) was lower in the supine position than in he left-tilted position (141 ± 48 ml/min/dl) (P < 0.01), while the myometrial blood flow was similar in both (9.0 ± 3.3 ml/min/100 g in supine and 8.7 ± 2.7 ml/min/100 g in left-tilted position). The results suggest that the autoregulation system for uterplacental circulation is operative only in the nonplacental component of the pregnant uterus.