Abstract: The dynamic changes in the activity of brain acetylcholinesterase (AchE) and the isozymes of three enzymes during the period of mud carp's cold shocking were preliminarily investigated through the methods of spectrophotometry and starch gel vertical electrophoresis. According to the remarkable lowering of brain AchE and the notable raising of the activities of lactate dehydrogenase (LDH) isozymes in the liver tissue, as well as the changes of the activities in malate dehydrogenase (MDH) and esterase(EST) isozymes in the tissues of mud carp which were under cold shocking, we discuss some biochemical factors correlated with the genetic trait of low temperature endurance in mud carp and suggest that the limiting activity of brain AchE could be used as a useful biochemical index showing the damage to the central nervous system, which in turn causes cold shocking and dying with metabolic disorders in the mud carp.