RÉSUMÉ

The authors studied the changes in contents of g-aminobutyric acid (GABA) and the intermediates involved in its forming pathway (shunt) in brains of pond carps during a year. An increased production of GABA and g-oxiaminobutyric acid (GOBA) promotes energy preservation in brain under unfavorable seasonal state offish organism (April). In NН₃ toxicosis, an increased GOBA production (GABA transamination) supports the tolerance of central nervous system to that toxicant. After addition of Mg²⁺ to water medium with an enhanced NН₃, glutamate, and GABA in brains; acetylcholinesterase activity was restored. These effects evidence a protective action of Mg²⁺ at stress states in fish.