Various substances considered as mediators responsible for regulation of both cellular and humoral components of inflammatory and allergic reactions are produced in course of oxidation of arachidonic acid by means of lipoxygenase. These substances included a number of stereochemically different monohydroxyeicosatetraenoic acids as well as leukotrienes. Components of slowly acting substance in anaphylaxis (leukotrienes C4 and D4) caused contraction of smooth muscles, constriction of bronchopulmonary system, alteration in permeability and tonus of capillary blood vessels in skin and other tissues. Leukotriene B and 5-monohydroxyeicosatetraenoic acid caused mobilization of neutrophils in the reaction of hypersensitivity of the delayed type. Contrary to prostaglandins, formed from arachidonic acid by cyclooxygenase, monohydroxyeicosatetraenoic acids and leukotrienes are considered as pathological agents only. Distinct and long-term effect of these substances on tissues suggest that the lipid mediators of inflammation will be important in studies of pathogenesis of various acute and chronic diseases.