The effects of ryanodine, a neutral alkaloid, on crab muscle fibers were investigated under voltage clamp conditions using the double sucrose-gap technique. In the presence of ryanodine, contracture develops without any membrane depolarization and the Ca-conductance variables are shifted in a hyperpolarizing direction. As a result, the Ca channels are activated at the resting potential. During the action potential, the Ca channels appear to open faster and for a longer period of time. The K currents are also modified. These different effects may be interpreted by a common mechanism related to a change of the membrane electrical field induced by increased Ca activity in the cytoplasm.