In solutions containing perchlorate in place of chloride frog toe muscles or small bundles of semitendinosus fibers undergo maximum potassium (K) contractures of long duration at low [K]0 (5–10 mM). After relaxation at high [K]0, large protracted "reactivation" contractures (70–90% of maximum tension) again develop when repolarization is accomplished by a reduction of [K]0 to 2.5–10 mM. Somewhat smaller contractures also appear during repolarization at lower perchlorate concentrations (8–12 mM). The effects of perchlorate include disproportionate shifts in the relation between log [K]0 and K-contracture tension, and between log [K]0 and relaxation rate. Similar but smaller effects are observed in the presence of 1 mM caffeine or 1.5 mM chloroform. These observations implicate at least two potential-dependent processes in the regulation of contraction in frog twitch muscle.