Ｏｂｊｅｃｔｉｖｅ　To investigate the transformation of field potential in the motor cortex of cat introduced by ketamine.　Methods　Three healthy adult male cats weighted 3.0 kg-5.0 kg were enrolled in this study. The changes of local field potential （LFP） power and the time-course of the power of δ （2 Hz-4 Hz）， θ （4 Hz-7Hz）， β （12 Hz-25 Hz） bands were assessed on neurons in right motor cortex（area 4γ） by in vivo extracellular record method under ketamine anaesthesia， accompanying with assessing energy ratios of four bands.　Results　Comparing with baseline values in awake rats， right motor cortex showed a fast increase in LFP power in the relevant bands after the 40 mg/kg ketamine dose， with maximum values achieved when intraperitoneal injection（i.p.） ketamine instantly， and then presented a gradual power decrease， reaching the power stable and superior to baseline values about from 6 min to 25 min after injection. The response of signal energy to the ketamine was characterized by a biphasic pattern with a very fast increase， followed by a sharp decrease about 6 min after i.p. ketamine and a gradual increase to a plateau superior to baseline values till the end of the recording. The slow-wave （δ， θ） energy increased higher than that of the fast-wave component （β） after anesthesia. The LFP was with more the fast-wave component （β） in awake cats， whereas cats were in anesthesia， the slow-wave components （δ） in LFP increased.　Conclusions　Ketamine has a profound effect on motor cortex LFP at the time of unconsciousness and anesthesia maintenance.