To examine the effects of repetitive bouts of heavy exercise on the maximal activities of enzymes representative of the major metabolic pathways and segments, 13 untrained volunteers (VO_2peak=44.3±2.3 ml.kg^-1. min^-1) cycled at ~ 91% for 6 min once per h for 16 h. Maximal enzyme activities (V_max, mol.kg^-1 protein .h^-1) were measured in homogenates from tissue extracted from the vastus lateralis before (B) and after (A) exercise at repetitions 1 (R1), 2 (R2), 9 (R9) and 16 (R16). For the mitochondrial enzymes, exercise resulted in reductions (P0.05) in cytochrome oxidase (COX, 14.6 %), near significant reductions in malate dehydrogenase (4.06% ; P=0.06) and succinic dehydrogenase (4.82 % ;p=0.09), near significant increases in -hydroxyacyl CoA dehydrogenase (4.94 %; P=0.08) and no change in citrate synthase (CS, 2.88 %; P=0.37). For the cytosolic enzymes, exercise reduced (P0.05) Vmax in hexokinase (HEX, 4.4 %), creatine phosphokinase (9.0 %), total phosphorylase ( 13.5 %), phosphofructokinase ( 16.6 %), pyruvate kinase (PK, 14.1 %) and lactate dehydrogenase (10.7%). Repetition-dependent reductions (P0.05) in V_max were observed for CS ( 1R,2R16R), COX ( R1,R2R16), HEX (1R,2R16R) and PK (R916R). It is concluded that heavy exercise results in transient reductions in a wide range of enzymes involved in different metabolic functions and that in the case of selected enzymes, multiple repetitions of the exercise reduce average V_max.