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Cromakalim, pinacidil, nicorandil, diazoxide and RP-49356 belong to the class of drugs termed potassium channel openers. In rat portal vein diazoxide, like cromakalim, abolished spontaneous mechanical and electrical activity and in rat aorta caused an increase in 86Rb efflux and inhibited KCl(20 mM)-induced contractions. However, in contrast to cromakalim, diazoxide (greater than 100 microM) also inhibited mechanical responses evoked by 80 mM KCl in rat aorta suggesting that it possesses pharmacological properties in addition to K channel opening. Since glibenclamide can attenuate the effects of cromakalim and diazoxide in vascular tissues, it is possible that a channel resembling the ATP-sensitive K channel found in pancreatic beta-cells may be involved in the vasorelaxant effects of these agents. However, differences exist in the order of potency of cromakalim and diazoxide for producing smooth muscle relaxation and for decreasing insulin secretion in pancreatic beta-cells. Furthermore galanin (which opens ATP-sensitive K channels in beta-cells) increases mechanical activity in rat portal vein. It is anticipated that new chemical developments will produce K channel opening molecules with greater potency and tissue selectivity.