Methanol, when introduced into all mammals, is oxidized into formaldehyde and then into formate, mainly in the liver. Such metabolism is accompanied by the formation of free radicals. In all animals, methanol oxidation, which is relatively slow, proceeds via the same intermediary stages, usually in the liver, and various metabolic systems are involved in the process, depending on the animal species. In nonprimates, methanol is oxidized by the catalase-peroxidase system, whereas in primates, the alcohol dehydrogenase system takes the main role in methanol oxidation. The first metabolite (formaldehyde is rapidly oxidized by formaldehyde dehydrogenase) is the reduced glutathione (GSH)-dependent enzyme. Generated formic acid is metabolized into carbon dioxide with the participation of H 4 folate and two enzymes, 10-formyl H 4 folate synthetase and dehydrogenase, whereas nonprimates oxidize formate efficiently. Humans and monkeys possess low hepatic H 4 folate and 10-formyl H 4 folate dehydrogenase levels and are characterized by the accumulation of formate after methanol intoxication. The consequences of methanol metabolism and toxicity distinguish the human and monkey from lower animals. Formic acid is likely to be the cause of the metabolic acidosis and ocular toxicity in humans and monkeys, which is not observed in most lower animals. Nevertheless, chemically reactive formaldehyde and free radicals may damage most of the components of the cells of all animal species, mainly proteins and lipids. The modification of cell components results in changes in their functions. Methanol intoxication provokes a decrease in the activity and concentration of antioxidant enzymatic as well as nonenzymatic parameters, causing enhanced membrane peroxidation of phospholipids. The modification of protein structure by formaldehyde as well as by free radicals results changes in their functions, especially in the activity of proteolytic enzymes and their inhibitors, which causes disturbances in the proteolytic-antiproteolytic balance toward the proteolytics and enhances the generation of free radicals. Such a situation can lead to destructive processes because components of the proteolytic-antiproteolytic system during enhanced membrane lipid peroxidation may penetrate from blood into extracellular space, and an uncontrolled proteolysis can occur. This applies particularly to extracellular matrix proteins.