Three metabolic pathways of ethanol in human body have been described thus far: 1) alcohol dehydrogenase (ADH), 2) microsomal oxidizing ethanol system (MEOS), 3), catalase. The main role in alcohol metabolism is being payed by cytosolic enzyme, alcohol dehydrogenase, which catalyses metabolism of ethanol to acetaldehyde. It occurs in many dimeric isoenzyme forms which are divided into three classes, according to substrate specifics and other chemical-physical properties. ADH is regulated by development and sex, its activity has been described in many tissues (liver, stomach, kidneys, ovaria, uterus, testes, retina, iris). In 1991 Yoshida found a new gene of alcohol dehydrogenase-ADH 6. 4-hydroxyalkenals, cytotoxic products of lipid peroxidation, are also substrates of ADH. Acetaldehyde is further metabolised by aldehyde dehydrogenase (AIDH) to acetate. AIDH occurs in the form of several isoenzymes in cytosole, mitochondria and microsomes of almost all tissues. Acetaldehyde thus binds to nucleic acids, phospholipids and especially to proteins, it may change their structure and function. In activates collagen synthesis and stimulates lipid peroxidation. Human body may be impaired by immune reaction caused by acetaldehyde binding proteins. Acetaldehyde has the function of a hapten. In 1968 Lieber and DeCarli described microsomal ethanol oxidizing system (MEOS). MEOS is one of cytochrome P450 isoenzymes, now called P450IIE1. Having a gene on chromosome 10, composed of 492 amino acids, this isoenzyme is present in cytoplasmatic reticulum. It is inducible and in case of chronic alcoholism, it accelerates elimination of ethanol from the human body. Inducted P450IIE1 is also present in Kupffer cells which participate in fibrogenesis of liver tissue and produce interleukin 1.(ABSTRACT TRUNCATED AT 250 WORDS)