Recently, a class of potassium (K) channels has been discovered which are regulated by the intracellular level of ATP. These channels have been termed ATP dependent K channels (KATP) and have been found to exist in the heart, skeletal muscle, pancreatic β cells, brain, and smooth muscle. In this article, we discuss the function of the KATP channel in the ischaemic myocardium and present evidence to suggest that activation of these channels may, on the one hand, result in a marked cardioprotective effect from reversible or irreversible electrical, functional or biochemical damage or, on the other hand, have the potential to produce electrical instability and a proarrhythmic effect. The therapeutic potential of potassium channel modulators is also discussed. Cardiovascular Research 1992;26:1011-1016