Specific binding sites have been demonstrated to exist in the heart for several drugs and hormones such as beta-blocking agents, cardiac glycosides, catecholamines, insulin, glucagon and acetylcholine. The specific binding sites for cardiac glycosides in the human heart have certain properties which make it likely that they are the pharmacological receptors for the therapeutic and toxic actions of digitalis glycosides: they are located in the cell membrane and bind cardioactive steroids reversibly with high affinity: half-maximal receptor binding occurs at approximately 2 nM (approximately 1.5 ng/ml) for digoxin; potassium decreases receptor affinity, calcium increases it; specific binding of ouabain, digoxin or digitoxin is related to inhibition of (Na+ + K+)-ATPase activity--which is supposed to be the receptor enzyme for cardiac glycosides. Human left ventricle contains approximately 1.5 x 10(14) binding sites/g wet weight, right ventricle approximately 0.9 x 10(14). In disease the number of receptors may decrease (hypothyroid states, myocardial infarction) or increase (hyperthyroidism, chronic hypokalaemia). Certain drugs (such as phenytoin) or different temperatures or pH changes cause a change in digitalis-receptor affinity. Thus, the number of receptors and possibly their properties are subject to regulation in clinically relevant situations. Further investigations will probably reveal those pathophysiological states, which allow the explanation of toxicity or digitalis refractoriness.