1. We have investigated the effects of the sulphonylurea, glimepiride, currently used to treat type 2 diabetes, on ATP-sensitive K(+) (K(ATP)) currents of rat cardiac myocytes and on their cloned constituents Kir6.2 and SUR2A expressed in HEK 293 cells. 2. Glimepiride blocked pinacidil-activated whole-cell K(ATP) currents of cardiac myocytes with an IC(50) of 6.8 nM, comparable to the potency of glibenclamide in these cells. Glimepiride blocked K(ATP) channels formed by co-expression of Kir6.2/SUR2A subunits in HEK 293 cells in outside-out excised patches with a similar IC(50) of 6.2 nM. 3. Glimepiride was much less effective at blocking K(ATP) currents activated by either metabolic inhibition (MI) with CN(-) and iodoacetate or by the K(ATP) channel opener diazoxide in the presence of inhibitors of F(0)/F(1)-ATPase (oligomycin) and creatine kinase (DNFB). Thus 10 microM glimepiride blocked pinacidil-activated currents by >99%, MI-activated currents by 70% and diazoxide-activated currents by 82%. 4. In inside-out patches from HEK 293 cells expressing the cloned K(ATP) channel subunits Kir6.2/SUR2A, increasing the concentration of ADP (1 - 100 microM), in the presence of 100 nM glimepiride, lead to significant increases in Kir6.2/SUR2A channel activity. However, over the range tested, ADP did not affect cloned K(ATP) channel activity in the presence of 100 nM glibenclamide. These results are consistent with the suggestion that ADP reduces glimepiride block of K(ATP) channels. 5. Our results show that glimepiride is a potent blocker of native cardiac K(ATP) channels activated by pinacidil and blocks cloned Kir6.2/SUR2A channels activated by ATP depletion with similar potency. However, glimepiride is much less effective when K(ATP) channels are activated by MI and this may reflect a reduction in glimepiride block by increased intracellular ADP.