The effects of cromakalim, a potassium channel activating drug, have been studied on isolated detrusor muscle strips from normal Landrace boar, normal and unstable mini-pig and unstable human bladder. Cromakalim abolished spontaneous activity in all strips but did not abolish the ability of the detrusor muscle from any of the specimens studied to respond to carbachol, increased extracellular K+ or transmural nerve stimulation. Intravenous infusion of cromakalim in the urethral obstructed mini-pig caused the characteristic unstable contractions associated with bladder outflow obstruction to be abolished, leaving the animal able to void. Experiments with potassium isotopes and the sucrose gap technique demonstrated that cromakalim increased the potassium permeability and hyperpolarised the cell membrane, consistent with its reported actions on other smooth muscles. These results suggest that drugs such as cromakalim, which act by reducing membrane excitability without inhibiting responses to existing innervation, may have a clinical application in the treatment of instability which is secondary to bladder outflow obstruction.