To characterize chorionic relaxation against hypoxia. To investigate the effect of acute hypoxia on placental vessels from patients with severely abnormal uterine and umbilical Doppler study versus control group. 20 and 53 chorionic vessels from abnormal and normal placentas respectively were dissected. The vessels were equilibrated in a Bicarbonate Krebs solution, continuously, bubbled with 95% O2. The vessels were contracted by an isotonic potassium Krebs solution, and were washed with normal sodium Krebs solution. After this initial regimen Thromboxane-mimetic U46619 was added to keep bath concentration of 10-7 M. After submaximal tension was achieved, vessels were exposed to hypoxia (95% N2 and 5% CO2) for one hour. The relaxation response to hypoxia was evaluated as the percent of change in the isometric force generated during hypoxia, compared with those developed during the U-46619 administration. Statistical analysis was performed. Relaxation was reduced in a 73% in abnormal group. Under hypoxic conditions there was a relaxation of 18.2%±1.5 SEM in normal group. In the abnormal group there was a relaxation of 4.9±1.6 SEM (p<0.05). A condition of chronic hypoxia, may play a role impairing the capacity of placental vasculature to relax in response to acute hypoxia. The mechanism involved in this phenomenon remains to be elucidated.