A novel ATP-sensitive potassium channel (K_ATP) channel agonist, BPDZ 154 (6,7-dichloro-3-isopropylamino-4H-1,2,4-benzothiadiazine 1,1-dioxide), was synthesized, and its effects on insulin-secreting cells were evaluated using electrophysiology, ⁸⁶Rb⁺ and ⁴⁵Ca²⁺ efflux, and RIA determinations of insulin secretion. BPDZ 154, an analog of diazoxide, inhibited both glucose-induced insulin secretion from isolated perifused islets and the secretion of insulin induced by glucose and tolbutamide. These effects were mediated by the activation of ATP-sensitive potassium channels because BPDZ 154 induced a concentration-dependent increase in channel activity that was inhibited by the sulfonylurea tolbutamide and the imidazoline efaroxan. In β-cells isolated from patients with either nontypical hyperinsulinism (preserved K_ATP channel function) or from the control areas of the pancreas of patients with focal hyperinsulinism, BPDZ 154 activated K_ATP channels and was found to be more effective and less readily reversible than diazoxide. By contrast, it was not possible to activate K_ATP channels by either diazoxide or BPDZ 154 in β-cells from patients with hyperinsulinism as a consequence of defects in K_ATP channel function. In β-cells isolated from a patient with pancreatic insulinoma, K_ATP channels were readily recorded and modulated by BPDZ 154. These data suggest that BPDZ 154 or BPDZ 154-like compounds may have therapeutic potential in the treatment of certain forms of hyperinsulinism.