OBJECTIVE: To investigate the effect of acute hypoxia on placental vessels reactivity from patients with severely abnormal uterine and umbilical Doppler study versus patients with normal Doppler indices. STUDY DESIGN: Isolated placental vessels (chorionic arteries and veins, 2 mm diameter) were obtained from patients with term pregnancies and abnormal uterine (S/D > 3.5 and notch) and umbilical (PI > 95 percentile) Doppler indices (placenta n -4, vessels, n = 20} and patients with normal Doppler study (placenta n= 11, vessels, n=53). Vessels incubated in Krebs bicarbonate buffer, under 95% 02 and precontracted with U46619, in an in vitro system were exposed to hypoxia (95% N2, 5% CO2) for one hour. Data is expressed as % of L'46619 steady state. ANOVA, Student-NewmanKeuls, and unpaired t test statistics were utilized. RESULTS: Relaxation capacity was reduced in a 73% in abnormal group. Under hypoxic conditions there was a relaxation of 18.2% ±1.5 SEM in normal group vs 4.9% ±1.6 SEM in abnormal group (p<0.05). There were not statistically significant differences between arteries and veins. CONCLUSIONS: Hypoxie relaxation of chorionic vessels in vitro is significantly reduced in vessels from placentas potentially with chorionic hypoxia in vivo. This abnormal response may be a possible explanation for a high impedance of umbilical-placental territory determined by Doppler studies.