A new "parametric" classification of the types of single-substrate enzymic reactions is proposed that includes 15 different types: seven inhibited. seven activated and one initial (uninhibited, i = 0 and non activated, a = 0). The new classification takes into account both the "parametricity" of these reactions and the mechanisms of their action. It unites all the types in an original symmetric system which vividly demonstrates the interconnection between separate (strictly definite) types of inhibited and activated enzymic reactions. The proposed classification permits the revision and some corrections to traditional "competitive" terminology as well as the application of these ideas and mathematical approaches pertinent to the calculation of reactions of en zyme inhibition for data analysis of enzyme activation. (C) 2002 Published by Elsevier Ltd.