In the present study,juvenile rainbow trout (Oncorhynchus mykiss) were acclimated at 5 temperatures.Fish in the con- trol group (C0) were reared at a constant temperature (16℃); trout in four other treatments (A0,A3,A6,and A9) were acclimated to a high temperature (22℃) for 0,3,6,and 9 d,respectively,and then reted to normal temperature (16℃) for 7 d.The temperature was then raised to 20℃ and the fish were cultured for 40 d.The results showed that the growth rates of A3,A6,and A9 were higher than that of A0 but lower than that of C0.The growth rate of A9 was the highest among the 4 acclimation groups.The activities of serum transaminase and liver antioxidant enzyme significantly increased during acclimation.At the beginning of growth,the activities of enzymes were lower in A3,A6,and A9 with the lowest in A9.In addition,the transcript abundance of heat shock protein (HSP) 60 gene in A9 was not significantly different from that of C0 during growth.HSP70 in A9 significantly increased at the beginning and reted to that of C0 at the end.Our findings indicated that pre-acclimation improved the high temperature tolerance with the best effectiveness observed at 22℃ for 9 d.A possible mechanism underlining such phenomenon is the improvement of antioxidant de- fense system.