HE effect induced by a drug is the resultant of the interaction between T the molecules of the drug and the molecules of which the biological object is composed. The higher the specificity required in the structure of the drug molecule, the more probable it becomes that the effect is based on an interaction of these molecules with certain specific molecules in the biological object. These specific molecules are called the receptors for the drug. They can be defined as those specific molecules, molecule complexes or parts of them in the biological object, with which the drug must interact in order to induce its effect. The term receptor goes back to Ehrlich (1913) who said: "Corpora non agunt nisi fixata". The notion of a specific receptive substance as a site of action for drugs such as nicotine and curare in the myoneural junction was introduced by Langley (1905). Since then the term receptor has become indispensable in reasonings on the basis of drug action. Often the model of key and lock is used for the drug-receptor interaction; reality is much more dynamic, however. Schueler (1960) defined the receptor as follows : "The drug-receptor is in general the pattern R of forces of diverse origin forming a part of some biological system and having roughly the same dimensions as a certain pattern M of forces presented by the drug molecule in such a way that between patterns M and R a relationship of complementarity for interaction exists." Drug-receptor interaction must be seen as a mutual moulding of drug and receptor. There is mutual adaptation as far as shape and charge distribution is concerned. This adaptation plays an important role in the activation of drug and receptor and therefore is essential to drug action.Drug-receptor interaction can have various consequences.(a) The drug-receptor interaction mainly leads to changes in the charge distribution and shape of the drug molecule, in such a way that it is activated and becomes chemically more reactive, which results in chemical changes in the drug molecule. The drug is metabolised. The receptor is the "active site" on an enzyme.(b) The drug-receptor interaction mainly leads to changes in the charge distribution on and in the shape of the receptor, so that as a result of this the receptor becomes activated and induces changes in the charge distribution and shape of the surrounding molecules, thus initiating the sequence of physico-chemical events leading to the effect.(c) The drug-receptor interaction may take place without essential changes in the drug molecule or the receptor. No effect is then induced.

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