OBJECTIVE To investigate the hemodynamics during extracorporeal membrane oxygenation (ECMO) and the value of transcranial Doppler (TCD) monitoring. METHODS ECMO was conducted on 14 sheep. TCD monitoring was conducted at different time-points to examine the bilateral cerebral blood flow velocity (CBFV), including systolic peak flow velocity (Vs), end-diastolic flow velocity (Vd), mean flow velocity (Vm), and pulse index (PI, Vs-Vd/Vm). The general condition, blood pressure, heart rate, and temperature were observed. Two weeks after, the surviving 9 sheep were killed, their brains were taken out and morphological and pathological examinations were done. RESULTS Anesthesia showed little effect on CBFV. After the ligation of carotid artery and vein, the CBFV pattern only changed slightly, Vs and PI decreased, and Vm and Vd remained almost unchanged. During ECMO with either greater or smaller volume, especially the former, the CBFV pattern showed a non-pulse waves, Vs markedly decreased with a value hardly different from that of Vd, Vs, Vd, and Vm were 62%, 75%, and 69% of the values in normal condition. The pre-ECMO CBFV was not significantly different from those examined any day after ECMO. The differences of pH and PO(2) in femoral artery during different stages were insignificant. PaCO(2) during ECMO with great flow volume and during mechanical ventilation after ECMO was significantly lower than that before ECMO (P < 0.05). The MABP of femoral artery during ECMO with small flow volume was significantly lower than that before ECMO (P < 0.05). Vm was positively correlated with MABP and not correlated with heart rate and temperature. The gross observation of the sheep's brain was normal. Small foci of malacia were seen in the brains of 4 sheep. CONCLUSIONS Hemodynamics changes remarkably during ECMO. TCD monitors the CBFV continuously during ECMO, thus helping maintaining the stability of CBFV and protect the brain.