Objective:To investigate whether qEEG can be a credible index to judge the depth of anaesthesia of ketamine(Ket).Methods:qEEG was recorded before and at 5min and 20min after iv injection of 4mg/kg of Ket in 9 adult female patients.Results:①REL:The α power was predominant on all leads before injection of Ket and the θ power was minimal,but at 5min after injection the θ power became completely dominant and lasted for more than 20min.②δ ratio:Before injectin of Ket δ ratio was little and at 5min after injection it was increased significantly (P<0 01).It began to decrease in 20min of injection,and even was the same as that before injection in F 7、F 8 leads.③θ ratio:θ ratio was low before injection and rose abruptly at 5min after injection and lasted at least for 20min.At the same time inverse θ ratio decreased abruptly.Conclusions:Durable high θ power is a character of a bolus Ket anaesthesia.The change of δ ratio can reflect the status of initial awakening from ket.θ ratio and reverse θ ratio reflect more accurately the status of anaesthesia of Ket than δ ratio.