In the study of the drug-receptor interaction, the analysis of the mode of drugantagonism had been the objects of the principal interests, and the drug-synergism had only been remarked in passing (1). Veldstra (2) mentioned the relation between drug receptor and synergism in his extensive review on “Synergism and Potentiation”. A fine definition of synergism and antagonism was proposed by Gaddum as follows. Figure 1 is some modification of that shown in his textbook. In the abscissa a multiple “m” of the effective concetration of the drug A and in the ordinate a multiple “n” of that of the drug B is shown. The synergistic effect is divided into the following manner : According to the classic Burgi's rule, it is said that, when two drugs, having similar pharmacological action and the same site of action, are used in combination, they are additive, and that, when the two drugs have different sites of action, they are superadditive. These phenomena could be elucidated by the application of mass action law to the interaction of drug-receptor combination, in a similar way as drug antagonism. As indicated in the title of this paper, our aim is to study precisely the relation between synergism and receptor theory by using isolated organs.