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Two usual assumptions of the treatment of metabolism are: (a) the rates of isolated enzyme reactions are additive, i.e. that rate is proportional to enzyme concentration; (b) in a system, the rates of individual enzyme reactions are not influenced by interactions with other enzymes, i.e. that they are acting independently, except by being coupled through shared metabolites. On this basis, control analysis has established theorems and experimental methods for studying the distribution of control. These assumptions are not universally true and it is shown that the theorems can be modified to take account of such deviations. This is achieved by defining additional elasticity-coefficients, designated by the symbol π, which quantify the effects of homologous and heterologous enzyme interactions. Here we show that for the case of non-proportionality of rate with enzyme concentration, (πii≠ 1), the summation theorems are given by

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The example of monomer-oligomer equilibria is used to illustrate non-additive behaviour and experimental methods for their study are suggested.