Functional parameters (adhesivity, permeability, thrombogenicity and thromboresistance) of the mesenteric microvasculature endothelium in rats bearing 1,2-dimethylhydrazine (DMH) induced colon tumors were studied using a complex method, based upon computer-assisted microscopic video image processing. It was shown that paraneoplastic endothelial dysfunction in rat mesenteric microvessels is characterized by an increase in leukocyte adhesion to the endothelium, microvessel permeability and thrombogenic properties. It was demonstrated that the observed changes are the consequences of tumor growth and are related to tumor size and the duration of the process.