1. Effects of cyclopiazonic acid (CPA), a specific inhibitor of the Ca(2+)-ATPase in sarcoplasmic reticulum (SR), on membrane ionic currents were examined in single smooth muscle cells freshly isolated from ileal longitudinal strips and urinary bladder of the guinea-pig. 2. Under whole-cell clamp, CPA (1-10 microM) reduced peak outward current elicited by depolarization in a concentration-dependent manner. The concentration of CPA required for 50% decrease in the peak outward current was approximately 3 microM in ileal cells under these conditions. The current reduced by CPA recovered by more than 70% after washout. 3. The transient outward current elicited by application of 5 mM caffeine at a holding potential of -50 mV in Ca2+ free solution was almost abolished, when the preceding Ca(2+)-loading of the cell in a solution containing 2.2 mM Ca2+ was performed in the presence of 3 microM CPA. 4. When the Ca(2+)-dependent K+ current (IK-Ca) and Ca2+ current (ICa) were inhibited by addition of Ca2+, the remaining delayed rectifier type K+ current was not affected by 10 microM CPA. When outward currents were blocked by replacement of K+ by Cs+ in the pipette solution, the remaining ICa was not affected by 10 microM CPA. 5. CPA (10 microM) did not affect the conductance of single maxi Ca(2+)-dependent K+ channels or the Cd(2+)-dependence of their open probability in both inside- and outside-out configurations. 6. These results indicate that IK-Ca is selectively and strongly suppressed by CPA.Its effects may be attributed to a decrease in Ca2"-uptake into SR, resulting in a decrease in Ca2"-induced Ca24 release which is triggered by Ca24 entering through voltage-dependent Ca24 channels and therefore less activation of these K channels.7. CPA may be extremely valuable pharmacological tool for investigating intracellular Ca24 mobilization and ionic currents regulated by intracellular Ca24.