<p>We studied the effect of Cu²⁺ on cultured endotheliocytes of the rat cerebral cortex. Phase-contrast microscopy of living cultures showed that cell damage in the monolayer and a decrease in the mitochondrial membrane potential (detected with JC-1 dye) occurred when the Cu²⁺ concentration in the culture medium reached 0.2 mM and higher. In surviving cells exposed to 0.2 mM Cu²⁺, an increase in the level of p53 protein in the nuclei was revealed. However, statistically significant decrease in the conversion of tetrazolium bromide into formazan was observed starting from 0.1 mM Cu²⁺ in the culture medium, and impairment of the barrier function of these cells began from 0.05 mM Cu²⁺.Thus, among the studied parameters, the barrier function of endotheliocytes was proved to be the most sensitive to the toxic effect of copper.</p>