Within 6 weeks after surgical isolation of a wide area of the rabbit neocortex EEG spectra were studied before and after acetylcholine application to the isolated cortex. Acetylcholine application initiated an intricate multiphase reaction in the sensorimotor and visual areas of the isolated cortex which lasted for more than 10 minutes. It consisted of an increase in EEG amplitude and progressive change in EEG rhythms, i. e., enhancement of the theta-waves and appearance of higher frequencies (up to 15 Hz) in the EEG. In the control series of experiments (rabbits with the intact neocortex) acetylcholine application produced an increase in EEG amplitude and enhancement of peaks in EEG spectrum. The response of the isolated cortex is interpreted as an artificially induced activation. The distinctions between responses of the isolated and intact neocortex are explained by retention of subcortical control over the state of the intact cortex.