ABSTRACT Receptors and ion channels are membrane-bound proteins involved in fundamental regulatory mechanisms in cells. They govern both a cell’s homeostatic activity and its programmed responses to intra- and extracellular stimuli. Specific receptor molecules in a cell’s plasma membrane can be activated by signal molecules present either in the medium surrounding the cell or even generated within the cell itself. The activated receptors sometimes directly bring about changes in the conductances of ion channels. On other occasions the receptors generate changes in a cascade of intracellular second messengers that subsequently activate the ion channels or effect appropriate biochemical changes. In many cases the receptors themselves are actually part of the ion channel complex and in other cases the receptor properties reside in distinct molecular entities. Ion channels may also be regulated by changes in the potential across the membrane in which they are found and also by changes in the concentrations of specific ionic modulators within the cell. Thus the actions of receptors and ion channels in the regulation of cellular function are inextricably intertwined.