1. Single smooth muscle cells from the guinea-pig taenia caecum and the fundus of guinea-pig stomach were prepared by collagenase digestion under different, mild conditions. 2. Most of the cells either from taenia caecum or from the fundus of stomach responded repeatedly, showing an all-or-none response to acetylcholine (ACh). 3. The threshold concentrations of ACh were different for the cells of the two tissues. Although individual cells showed an all-or-none response to ACh, the average responses of all the cells were graded, like that of whole tissues. 4. Thus, isolated single smooth muscle cells from different tissues and under different conditions responded to ACh in an all-or-none manner such as the twitch observed in skeletal muscle. 5. These results suggest that the isolation of cells reveals the fundamental characteristics of smooth muscle cells as excitable.