The purpose of the present work was to study the effects of tolerance to ethanol on the sleep-wakefulness cycle (SWC) structure. Experiments were carried out in chronic conditions on the adult cats (n=5). The following methods were used: the stereotaxic - for implanting electrodes; polysomnographic - for EEG registration of the SWC structure. Alcoholisation (0.2-2.5 g/kg 25% ethanol solution) was conducted by intraperitoneal injections that lasted for two weeks. The obtained results were processed statistically and significance of the changes was determined by the Student t-test. Low single doses of ethanol (0.2 - 0.5 g/kg) did not induce any significant changes in the structure of the SWC. While using doses of 0.6 g/kg it was noted only increasing of the latent period of the onset of sleep. However, the structure of the SWC recovered within 1-2 hours after injection. Increasing the dose to 1 g/kg caused severe intoxication. This was reflected in a behavioral (anxiety, tremor, vocalizations) and autonomic (vomiting and frequent urination) signs. Against the background of restless behavioral wakefulness synchronization of the characteristic of light slow-wave sleep developed. The volume of deep slow-wave sleep was statistically significantly decreased and the latent period of the onset of paradoxical sleep was increased. The structure of sleep was fragmented, due to frequent awakenings. For the fifth-eighth day of alcoholization the structure of the SWC restored, behavioral and vegetative signs of intoxication disappeared, which must indicate the development of tolerance to this dose of ethanol. The obtained results signify that development of tolerance to ethanol can be reflected in the alteration of the structure of SWC. Alcoholization disturbs the subtle mechanisms of the sleep due to destruction of brain homeostasis.