The contractures induced by 20-200 mM [K+]o in single crab muscle fibers were resolved into two components. The first component, consisting of single twitches or brief tetanic contractions, was associated with electrogenic membrane responses. The second occurred after spiking subsided with an amplitude that increased linearly with the [K+]o between 20 and 90 mM. The amplitude and time course of the contractures elicited by a given [K+]o differed markedly between different fibers. Contracture reproducibility of a single fiber was best when 90 mM [K+]o was used. The K-induced contractures were abolished after brief (3 min) exposure of the fibers to a calcium-free solution and were greatly depressed by 8 mM procaine. The data suggest that the contractures require both Ca2+-influx across the sarcolemma and release of Ca2+ stored in the sarcoplasmic reticulum.