In order to make clear the activating effect of MCI-2016 on EEG, power spectrum analysis was performed and the interaction in EEG between MCI-2016 and some drugs were studied in succinylcholine-immobilized cats. MCI-2016 at doses of 5 and 10 mg/kg iv produced apparent arousal pattern in cortical EEG characterized by low amplitude fast wave, and evoked the right-shift in power spectrum. In the case of meclofenoxate at doses of 20 and 40 mg/kg iv, the shifts in power spectra were similar to that of MCI-2016, very slight changes were observed by Ca-hopantenate at a dose of 200 mg/kg iv and typical left-shift was introduced by imipramine at a dose of 2 mg/kg iv. MCI-2016 tended to suppress the SWS state induced by alpha-methyl-p-tyrosine and scopolamine at a dose of 350 mg/kg ip and 0.015 mg/kg iv, respectively. Subsequently, the duration of arousal state in EEG induced by physostigmine at a dose of 0.01 mg/kg iv was enhanced by MCI-2016 at a dose of 1.5 mg/kg iv whose injection does not produce any arousal pattern in EEG. The activating effect of MIC-2016 on EEG was indicated quantitatively and it is suggested that the cholinergic mechanism and the catecholaminergic mechanism would be involved in the EEG activating effect of MCI-2016.