Electrical and mechanical activities in high-K-induced contracture of guinea pig ureter and taenia coli were studied using the sucrose-gap method. Ureter immersed in high-K solution showed a contracture consisting of three components as well as twitch contractions evoked by a few spikes which were observed during the depolarizing phase. Reapplication of high-K solution after a short washing procedure with normal solution induced a contracture composed of only the first and the third components. No action potential was observed during the course of membrane depolarization by this procedure. High-K-induced contracture was highly sensitive to extracellular Ca: no tension development of the ureter despite the membrane depolarization was observed in Ca-free high-K solution. The addition of Ca to Ca-free high-K solution caused a contracture similar to that by high-K solution in the presence of Ca. Verapamil (10(-5) M) blocked all three components, leaving only slightly depressed twitch contractions initiated by action potentials. The possibility that all of these components of high-K-induced contracture were initiated by influxed Ca from extracellular space was also strengthened.