Organic anion transporter proteins (OATР) are members of the Na-independent transport system that at physiological values of рН transport substantial amounts of both endogenous (bile acids, eicosanoids, steroid and thyroid hormones, etc.) and exogenous (anionic oligopeptides, organic dyes, various toxins, drugs, etc.) amphiphilic organic anions with molecular masses greater then 350 Da through plasma membranes. These transporters are essential for the xenobiotic transformation system and contribute greatly to its functioning through a tight interrelation with the enzymes of phases I and II of xenobiotic detoxification, which can be performed through alteration of metabolic pathways due to simultaneous modification of the activity of enzymes and transporters. The drugs targeting the enzymes of phases I and II can often act as substrates or inhibitors for the transporters. At present, the structure of around 300 polypeptides from nearly 40 animal species has been decoded either completely or partially. Based on the similarity of the amino acid sequence proteins are grouped into families (up to 40 % similarity) and subfamilies (up to 60 % similarity). The phylogeny of ОАТР4В1 indicates that this protein has a very close evolutionary distance between mammals and other species. Being found in all investigated species, transporters of the subfamily ОАТР1С are considered the most evolutionarily ancient. No OATP homologue have been found in plants, yeasts and bacteria.