Аннотация:Small mammals were trapped in the western part of West Sibirian Lowland. Mean brain weight in voles (Clethrionomys glareolus, C. rutilus, Microtus oeconomus, and M. gregafis) and shrews (Sorex araneus and S. minutus) were maximum in summer and declined during autumn and winter. This decline was caused both by changes in population structure and mainly by brain weight decrease in separate individuals. During the period of"growth jump" in spring, the brain weight of small mammals began to increase again. Winter reduction of brain weight was determined mainly by water losses, and by the decrease in the dry rest weight as well. The most intensive decline in weight was observed for the telencephalon (neocortex especially, 22% in bank voles and 35% in common shrews). Weights ofsuch brain parts as bulbus olfactorius and myelencephalon did not change significantly during winter. Weight reduction ofthe telencephalon was accompanied by some cytomorphological changes in its architectonic.