Six weeks after surgical isolation of an extensive region of the rabbit neocortex, chronic experiments were carried out to study the EEG spectrum in baseline conditions and after macroapplication of acetylcholine (ACh) to the isolated cortex. Application of ACh elicited a complex, multiphasic response in the sensorimotor and visual regions of the isolated cortex, which lasted more than 10 min and consisted of an increase in the EEG amplitude with a gradual alteration in rhythmicity: there was an increase in theta activity, and higher-frequency (up to 15 Hz) processes appeared in the EEG trace. In control experiments (rabbits with intact cortex), application of ACh increased the EEG spectral amplitude and peak height. The response of the isolated cortex was interpreted as artificially induced activation, the level of activation determining the EEG spectrum. The differences in the responses of the isolated cortex and intact cortex result from preservation of control of the intact cortex by subcortical structures.