A potassium (K) channel that was inhibited by physiological (f1M) concen trations of intracellular ATP ([ATP] i) and that opened as [ATP] i decreased was first described in the heart (l). Subsequently, similar K-channels all with unitary conductances in the range of 40-80 pS (measured under symmetrical high K+ conditions) were also found to exist in insulin-secreting cells and in skeletal muscle (2_5). 1 Such channels constitute what are classically described as ATP-sensitive K-channels and were termed Type 1 by Ashcroft & Ashcroft (6). In this review they are designated KATP and the current flowing through them is defined as IK (ATP). Type 1 KATP channels are essentially calcium-and voltage-independent, K+-selective, and are half-maximally inhibited by [ATP] i in the range 10-100 fJ. M. They exhibit inward rectification and a key pharmacological feature is their inhibition by agents like tolbutamide and glyburide. Other ATP-sensitive K-channels exist and these were designated Types 2, 3, 4, and 5 (6). Such channels vary not only in their sensitivity to calcium and to the inhibitory effects of [ATP] i, but also in their selectivity for potassium and their susceptibility to pharmacological modulation.