This study was performed to investigate the action of potassium channel openers on the mechanical activity of detrusor muscle isolated from rats. Detrusor muscle strips, 15 mm in length, were myographied isometrically in an isolated organ bath. P 1060, RP 49356 and BRL 38277, potassium channel activators, reduced the basal tone and diminished the phasic activity of detrusor concentration-dependently. P 1060, RP 49356 and BRL 38227 suppressed the maximal responses to bethanechol and shifted the concentration-response curves of bethanechol-induced contraction to the right. RP 49356 and BRL 38227 reduced the contraction by low (20 mM) concentration of potassium. P 1060, however, diminished the high (80 mM) and low (20 mM) concentration of potassium-induced contraction. Glibenclamide, an inhibitor of ATP-dependent potassium channel, antagonized the suppressive action of P 1060, RP 49356 and BRL 38227 on the basal tone. Apamin or procaine did not antagonize it significantly. Based on these results, it is suggested that the relaxation of detrusor muscle strip caused by P 1060, RP 49356 and BRL 38227 may predominantly involve opening of the same potassium channel, i.e., the ATP-dependent potassium channel.