To observe the effects of propofol or thiopental on cerebral blood flow, twenty-two patients scheduled to undergo neurosurgery, ASA graded I to Ⅱ,aged 20 to 58 years,were assigned randomly to receiving a intravenous bolus of either 2. 0mg· kg-1 propofol (group P, n=12) or the equivalently effective dose of thiopental 4.0mg. kg-1 (group T,n=10 ). In condition that the level of PetCO2 was kept in the normal range by a ventilator, peak, mean and end-diastolic blood flow velocity(Vp, Vm, and Vd) of middle cerebral artery(MCA) were measured by the transcranial Doppler sonography at 5 mins before and 10 mins after the administration of propofol or thiopental, meanwhile the mean artery pressure (MAP) was monitored. The values of the pulsatile index (PI) and resistance index(RI) of MCA were given by the following equations: PI=(Vp-Vd)/Vm and RI =(Vp-Vd )/VP. The results showed that following the administration,the levels of MAP,Vm,Vp,Vd and PI significantly decreased separately by 13%, 24%, 31%, 20% and 29% in group P (P<0. 01 ) and by 9.7%, 13%,22%,21% and 14% in group T (P<0.01),and the value of RI was markedly reduced by 26% (P<0. 01) but remined stable in group T. There were not statistical differences in the above parameters apart from RI between two groups. Therefore,it is considered that the effects of propofol on cerebral blood flow may be similar to those of thiopental.