Incalcium-freesolution, strips of cat intestinalmuscledeveloped slow, rhythmic electrical potential changes that triggered contractions. Some strips failed to develop spontaneous electricalactivityincalcium-freesolutionbut responded with contractions todepolarizationby direct electrical stimulation or by treatment with barium chloride, potassium chloride, or acetylcholine. Similar results were obtained with segments of cat stomach, colon, esophagus, bladder, uterus, and vena cava, as well as with rabbit vena cava. Incalcium-freesaline, rat small intestinalmuscleshowed fast electricalactivitywith accompanying development of a tetanuslikecontraction. After 60 min incalcium-freesolution, cat small intestinalmuscleretained 17.7% of its original concentration ofcalcium. It is concluded that in somesmoothmuscles,depolarization-triggered release of intracellularcalciumdoes not require an associated influx ofcalcium.