In single smooth muscle cells held at depolarized potentials under voltage-clamp, spontaneous transient outward currents (STOCs) of about 100 ms in duration and 100-300 pA in size are observed. These behave as if they are caused by the cyclical release of calcium from storage sites within the cell and can therefore be used as a method of monitoring the release of calcium from these stores by various agents. Muscarinic receptor activation or caffeine applied to single smooth muscle cells from rabbit jejunum produced outward current, probably due to the release of calcium from stores, and STOCs were subsequently abolished. Levels of inositol 1,4,5 trisphosphate in fragments of the muscle were increased within 5 s by carbachol application. STOCs and outward current due to calcium store release were probably due to the opening of calcium-activated K-channels since they were abolished by 4 mM tetraethylammonium. Receptor activation also opens channels that admit cations, including calcium, which may also contribute to tension generation.