In this article the effect of 36 h total sleep deprivation (TSD) on behavioural reactions of rats (rearing, grooming and sexual activity) and on ultrastructure of brain neurons have been studied. A group of somnogenic structures (III-V layers of the frontal limbic cortex, CA1 area of the dorsal hippocampus, reticular formation of pons varolli, nucleus raphe dorsalis and locus coeruleus) of the brain of 6 month-old Wistar rats has been analized in this study. It has been revealed that dystrophic changes (chromatolysis of cytoplasm and vacuolization) enveloping a group of medium-sized neurons (20-30 (μ) micron in diameter) are accompanied by decrease of numbers of rearing, grooming and sexual activity. It is supposed that dystrophic changes revealed in medium-sized neurons result in partial disturbances of integrative and behavioral reactions of animals.