1 A method is described for deriving null equations for functional antagonism and synergism. These null equations relate together the concentrations of agonist required to produce equivalent states of a cell or tissue in the presence and in the absence of a functional interactant. 2 In one particular case the null equation leads to conclusions which are very similar to those reached by an earlier author who did not use the null method. However, the null equations give a clearer insight into the quantitative aspects of functional interaction. 3 It is concluded that the use of functional antagonism to estimate affinity constants and relative intrinsic efficacies of agonists has serious limitations. On the other hand, it may be possible to use the null equations, or similar principles, to test the validity of postulated mechanisms and sites of action of functional interactants.