Experiments have been carried out to examine the effects of GTP on the opening of K+ channels in insulin-secreting cells by diazoxide (0.2 mM) and cromakalim (0.5 mM). Using rat pancreatic beta-cells and RINm5F insulinoma cells, patch-clamp recordings of unitary ATP-sensitive potassium (K+ATP) channel currents were made in either the open cell or outside-out patch recording configurations. Adding diazoxide or cromakalim to either the inside or the outside face of the membrane was found regularly to cause the activation of K+ATP channels in the presence of 0.5 mM ATP. We now demonstrate that in the absence of ATP but in the presence of GTP (0.5-1 mM), both diazoxide and cromakalim activate channels. Effects are rapid in onset, sustained and readily reversible. Both the diazoxide- and cromakalim-induced activation of K+ATP channels were mediated by increases in channel open-state probability, and were not associated with any significant change in either channel amplitude or by an increase in the number of channels in the patch. The actions of both diazoxide and cromakalim were not affected by overnight pretreatment of cells with pertussis toxin, suggesting that PTX-sensitive GTP-binding proteins are not involved in mediating the actions of either compound. These data indicate that diazoxide and cromakalim open K+ATP channels in a manner not solely dependent upon intracellular ATP, but by mechanisms involving other cytosolic nucleotides, including GTP.