The cell-surface receptors that combine with hormones or neurotransmitters to initiate cellular responses are a heterogeneous group of proteins with specific recognition sites for binding particular compounds with a high affinity and specificity. These molecules are relatively mobile in the cell membrane and, when occupied by the appropriate hormone or neurotransmitter, can interact with other effector molecules within the membrane to alter ionic gradients, enzyme activity or phospholipid metabolism, thus initiating a biological response. The relationship between occupancy of the receptor by hormone or neurotransmitter and production of a cellular response is not the same in every tissue and can be altered by various mechanisms when the normal stimulation of the tissue is increased or decreased over a period of time. Such alterations in sensitivity are important in the pathology of a number of disease states and also important in long-term adaptation to chronic drug therapy. For these reasons, an understanding of the molecular basis of drug action and the receptor molecules through which drugs act is important to the rational use of drug therapy.