The flexor carpi radialis muscle (FCRM) of the frog was divided into phasic and tonic bundles, and the properties of the potassium contracture in the tonic bundle were examined.The potassium contracture was tonic and the contracture induced by K higher than 75mM consisted of the initial phasic component and of the following sustained component. Both components were abolished by the transverse tubule disruption. The curve relating the peak tension to the log [K] _o (activation curve) started at about 15mM K and reached maximum at about 75mM K. By reducing [Ca] _o, the activation curve shifted downward at higher [K] _o, being little affected near the mechanical threshold.The time course of inactivation induced by 20mM K was slow and monophasic, and was markedly accelerated by reducing [Ca] _o.The spontaneous relaxation of the contracture induced by high K was little affected by reducing [Ca] _o.