Vasopressin was shown to cause a concentration-dependent relaxation of human basilar [correction of bacillary] artery's smooth muscles. In old age, the reactivity of these muscles in decreased and tachyphylaxis to vasopressin disappeared. This seems to be connected with a decrease in the quantity of smooth muscle's V2-inhibitory receptors. Protein kinase C activation does not affect molecular mechanisms of vasopressin action on human basilar [correction of bacillary] artery's smooth muscles. The data obtained suggest vasopressin participation in regulation of human cerebral blood flow and a limitation of adaptive properties of brain vessels' smooth muscles in old age. These factors, apparently, play a role in development of the brain circulation pathology in old men.