The critical thermal maximum (СTM) of the fingerlings of river perch and yearlings of carp are investigated. The range of heating rates of from 0.08 to 46 ° C/h. The values of the two-year carp from KTM for all heating rates were higher than the two-year perch, and the maximum temperature upheaval and loss of coordination of movements observed in carp at the slowest rate, the perch – at the very slow and fast heating rates. The brain enzyme acetyilcholinesterase activity (АChE) and soluble protein content (SPC) of the fishes, subjected heating is determined. Changes of activity АCh and contents SPS are discussed at different rates of heating. On the basis of data on the activity of AChE and content in CPC, one can conclude that in the range of heating rates up to 8? C / h by fish quite easily adaptable to increasing ambient temperature. However, further increasing the heating rate leads to increased cholinergic biosynthetic processes in the brain of the fish, which is reflected in increased activity of AChE and protein content. It is assumed that increasing values of both parameters at a heating rate of 16 and 32 °C/h is the stress response of fish in response to changing ambient temperature. It is possible that the performance of AChE and PCB could potentially be used as a biological marker of exposure to heat different types (natural environment, pre-acclimation, and heating or cooling). Thus, the research activity of AChE and VRB in the brain content of fish at different rate water heating allows to re-evaluate how adaptation possibilities, and physiological and biochemical mechanisms of the reactions of aquatic animals in the course of the temperature adaptations.