Pharmacological effects of lemakalim on cardiovascular system were investigated using isolated rat hearts and conscious SHRs subjected to hyperkalemic and hypokalemic condition. In the isolated hearts perfused with normal physiological salt solution(4.7 mM KCI), lemakalim increased cardiac function and coronary flow, and these effects were significantly potentiated under hypokalemic(1.2, 2.5 mM KCI), but attenuated under hyperkalemic(IO mM KCI) condition. In conscious SHRS, lemakalim(0.1, 0.2, 0.3mg/kg, p.o.) produced a dose-related decrease in systolic blood pressure, the maximal hypotensive effect being reached around 0.5 hr after dosing. The intensity and the duration of hypotensive effect of lemakalim were significantly increased when administered in combination with dihydrochlorothiazide (2 mg/kg, p.o.), but decreased with triamterene(32 mg/kg, p.o.). It appears that the differential effects of two types of diuretics on the hypotensive action of lemakalim are due to their hypokalemic and hyperkalemic action, respectively. It is conclued that the concomitant use of $K^{+}$ channel openers and hypokalemic diuretics may be an appropriate model of combination therapy in the treatment of hypertension.