1. It has previously been shown that the hypoglycaemic drug (-)-AZ-DF-265 stimulates insulin release from mouse islets; in the presence of 3 mM glucose it also inhibits 86Rb-efflux from 86Rb-loaded islets. Based on these data we tested the hypothesis that (-)-AZ-DF-265 inhibits ATP-sensitive potassium channels. 2. We voltage-clamped the plasma membrane of single rat pancreatic beta-cells in the whole-cell configuration and measured the current flowing through ATP-sensitive potassium channels. (-)-AZ-DF-265 was applied to the outside of the cell; it inhibited the current half-maximally at a concentration of 1.2 +/- 0.2 nM with a Hill coefficient of 0.7 +/- 0.1. The inhibition was reversible on washing. 3. In intact RINm5F cells, the sulphonylurea [3H]-glibenclamide bound with an affinity of 0.24 +/- 0.01 nM and a Hill coefficient of 2.1 +/- 0.4. Treatment with (-)-AZ-DF-265 led to the displacement of [3H]-glibenclamide; this effect was half-maximal at 8.6 +/- 1.7 nM and displayed a Hill coefficient of 0.53 +/- 0.01. Meglitinide and tolbutamide, which represent, respectively, the benzamido and sulphonylurea moieties of glibenclamide, showed Hill coefficients of 0.8 +/- 0.1 and 0.9 +/- 0.1, respectively. 4. At pH 7.4 and with phosphate/borate buffer, (-)-AZ-DF-265 had an apparent octanol/water partition coefficient of about 53, which is intermediate between the partition coefficients of glibenclamide and glipizide. Nevertheless, (-)-AZ-DF-265 bound only to a minor degree to glass and plastic.5. In conclusion, (-)-AZ-DF-265 inhibits ATP-sensitive potassium channels and displaces [3H]-glibenclamide from the sulphonylurea receptor.