Cypermethrin is synthetic pesticide that has been in use for more than a decade. A study of a low-dose exposure of this pesticide in rat administered, followed by determination of enzyme rate constant will highlight early catalytic changes that generally accompanied a toxic response in the animal. Even though, cypermethrin was used at 5 mM, for durations of 6, 12 and 24 h in the different groups of rats, results of the study indicate that simultaneous with changes in enzyme activity and rate constant of selected enzymes in selected tissues and serum. Tissue specific changes in the enzyme rate constant were suggestive of the lack of tissue resistance to pyrethroids.  Key words: Cypermethrin; ALT; AST; ALP; ACP; GGT