The longitudinal body wall muscle of Sclerodactyla briareus was studied in terms of the ionic bases of muscle contraction. Depolarization of the plasma membrane by acetylcholine requires sodium ions, while depolarization by potassium chloride does not. Calcium ions involved in excitation-contraction coupling come from two sources: influx from the extracellular medium, and release of the intracellular stores by two mechanisms: calcium induced calcium release and depolarization induced calcium release. Contraction in response to 50 mM KCl depends mainly on calcium influx and partially on calcium released by membrane depolarization. Contraction in response to acetylcholine depends on calcium influx and on the release of stored calcium by both mechanisms. Caffeine contracture involves calcium released by calcium.