In order to investigate the effect of prolonged exercise training on swimming performance and the underlying biochemical mechanism in juvenile common carp (Cyprinus carpio),we measured the critical swimming speed (Ucrit),excess post-exercise oxygen consumption (EPOC),the activity of red and white muscle enzymes [pyruvate kinase (PK),lactate dehydrogenase (LDH) and citrate synthase (CS)],the tissue substrates (glycogen and glucose contents of muscle and liver) and metabolites (the lactate contents of plasma and muscle) contents of exercise trained (60% Ucrit for 4 weeks) and non-trained fish.We also measured the biochemical indices of both trained and non-trained fish immediately after Ucrit and exhaustive exercise and 1 hour after exhaustive exercise.The aerobic swimming performance as indicated by Ucrit increased significantly after exercise training possibly due to the higher tissue metabolic capacity as suggested by higher CS activity in red muscle,higher energy store and more efficient substrate utilization as suggested by higher liver and muscle glycogen contents at rest but lower tissue glycogen contents after Ucrit.The lower lactate content after Ucrit possibly due to higher aerobic metabolic capacity and (or) clearance rate of lactate in trained fish may also contribute to improved aerobic swimming performance.Compared to Ucrit,exhaustive exercise elicited higher plasma and muscle lactate contents.The anaerobic metabolic performance is not affected by the exercise training as suggested by the EPOC.However,trained fish did show higher lactate clearance rate as suggested by lower muscle lactate content 1 h recovery after exhaustive exercise compared to non-trained fish.Furthermore,trained fish decreased their liver and muscle glycogen contents more profound after exhaustive exercise,suggesting training can improve the substrate utilization during anaerobic exercise.