Abstract
1. Frog toe muscles and isolated fibres from frog semitendinosus muscles were allowed to develop maximum K-contractures, and after reaching peak tension were transferred to media containing intermediate [K](0) (20-40 mM-K). Under these circumstances, relaxation displayed an early rapid phase, and a subsequent slower phase whose magnitude and rate varied with [K](0). The removal of activator (calcium ion) from the sarcoplasm appears to have a potential-dependent component.2. The relation between relaxation and [K](0) suggests that, with progressive depolarization, membrane sites from which calcium is initially released become converted into sites which are again capable of binding calcium and removing it from the sarcoplasm.3. The rate of relaxation of frog toe muscle after maximum K-contractures can be either accelerated or retarded by abrupt alterations in [Ca](0) or by sudden replacement of the major extracellular anion. These effects are attributed to shifts in the relation between calcium rebinding and membrane potential.