1. Spectrophotometric techniques were used to monitor the changes of enzyme activities in liver and muscle tissues of grass carp, mud carp, mixed sperms (grass carp's & mud carp's) inseminated mud carp and control mud carp after thermal acclimation. The concentration of lactate dehydrogenase (LDH) and succinate dehydrogenase (SDH) were higher, and isocitrate dehydrogenase (IDH) and glucose-6-phosphate dehydrogenase (G-6-PDH) were lower in liver tissues of grass carp acclimated to 10 ℃ than in he corresponding tissues of 25 ℃-acclimated grass carp, which indicating that some metabolic reorganizations must be occurred in liver tissue of grass carp during cold acclimation. All four enzyme concentrations in liver and muscle tissues of mud carp, mixed sperms inseminated mud carp and control mud carp, and muscle tissue of grass carp increased after cold acclimation, indicating that increasing enzyme concentration was the major or only method in liver and muscle tissues of mud carps and muscle tissue of grass carp during cold acclimation. Author be-lieved that the mechanism of metabolic reorganizations is more effective than the way of increasing enzyme concentrations only during cold acclimation, and probably this mechanism is not prefect in the tissues of mud carps. This may be the reason why mud carp could not tolorate low temperatures lower than 7 ℃. 2. Electrophoretic techniques were employed to study the changes of isozymes in tissues of fish acclimated to different temperatures. There was no significant difference observed in superoxide dismutase (SOD) and LDH isozyme patterns of any tissues in fish after thermal acclimation. The changes in the isozyme patterns of esterase (EST) in liver tissues of mud carp, mixed sperms inseminated mud carp and control mud carp indicated that some changes must be occurred in lipid metabolism of liver tissue of mud carps during thermal acclimation. 3. The GC/MS technique was applied to analyse the composition of fatty acids from mitochondria in muscle tissues of fish acclimated to different temperatures. Significant changes of fatty acid compositions were observed in muscle mitochondria of grass carp acclimated to 25 ℃ and 10 ℃. Compared with warm-acclimated grass carp, the ratio of short chain fatty acids (16C & 18C) decreased, and that of long chain polyunsaturated fatty acids increased in muscular mitochondria of cold-acclimated grass carp, and the average fatty acyl chain length increased to 20.46C from 19.08C. But little changes of the composition of fatty acids in muscular mitochondria of mud carp, mixed sperms inseminated mud carp and control mud carp acclimated to different temperatures were seen. In the present experiment, the contents of cholesterol in muscular mitochondria of warm- and cold-acclimated fish were determined, and the contents of cholesterol in all of them decreased after cold acclimation. The fluidity of mitochondrial membranes in muscle tissue of cold acclimated grass carp increased as a result of the changes of composition of fatty acids and the decreasing of the content of cholesterol. In the muscle tissues of cold-acclimated mud carp. mixed sperms inseminated mud carp and control mud carp, although the content of cholesterol decreased, increasing of fluidity of the mitochondrial membranes were limited because of the changes in composition of fatty acids were little. The difference of fluidity changes of the membranes between grass carp and mud carp are partly responsible for the difference of cold-endurance in these two fishes. 4. In this paper, the reasons for mixed sperms inseminating method improving the capacity of low temperature tolorance in mud carp were discussed. Author thought that mixed sperms inseminating method could improve the capacity of low temperature tolorance, but the improvement was not complete. Some suggestions of increasing the cold-resistance of mud carp were discussed.