RESUMEN
Excretion of metabolites is a characteristic feature of any alive organism. A big group of these products--second metabolites--because of their variability, quantity and physico-biological activity have a special importance in aquatic environment. Exometabolites of some organisms become an important part of environment for the others. The authors discussed the origin and evolution of exometabolites from simple waste products to biologically active substances. Quantitative and qualitative aspects of metabolic excretion by organisms in water conditions are analysed. The data on composition, origin and biological function of some second metabolites of different groups of aquatic organisms are presented. The authors propose a classification of second metabolites according to their functional significance. The role of metabolites and decay products in the development of chemical information streams in hydrobiocoenosis is analysed. Metabolites (soluble organic substances) form a field of chemical information for biotic community. The most important functions of this field are communication and conditioning. The authors emphasize the importance of investigations of chemical bioinformation field in aquatic ecosystems.