Potassium (K) channel openers comprise a diverse group of molecules capable of opening K channels in excitable cells. These agents exhibit their greatest potency in the smooth muscle system but K channels in cardiac muscle, neurones and in secretory cells are also affected. The development of tissue selectivity is currently one major focus of research and evidence is starting to emerge that this can be achieved. The profound effects of the K channel openers in vivo has led to the suggestion that an endogenous K channel opener might exist and exert an important role in blood pressure homeostasis. The discovery of such a substance--endothelium-derived hyperpolarizing factor--has many implications and its role in cardiovascular regulation is currently under investigation. In vivo, initial studies with the K channel openers emphasized their antihypertensive properties. However, later studies have concentrated on the improvement to coronary blood flow produced by these substances together with their protective effect on the ischaemic myocardium, the basis of which is not fully understood. In spite of great efforts, the K channel which forms the target of these agents in smooth muscle is a matter of controversy. The ability of glibenclamide to antagonize the actions of the K channel openers initially led to the suggestion that an ATP-dependent K channel was their site of action in smooth muscle although the most recent data have implicated a smaller conductance K channel.