The effect of altered external calcium concentration on potassium-induced contractures in single twitch muscle fibres of Xenopus laezis was studied. It was found that an increase in [Ca] shifted the curve relating peak tension to membrane potential to lower potential values and that the rate of relaxation was decreased. Decreased [Ca]° shifted the peak tension-membrane potential curve in the opposite direction and made relaxation quicker. The relation between inactivation of the contractile system and membrane potential was also affected by changes in [Ca]°; increased [Cal]° shifted the curve towards more positive potential values and decreased [Ca]° caused a large shift in the opposite direction. Small effects of altered [Ca]° were seen on the recovery of twitches after a preceding contracture in 190 m M-K The refractory period was shortened by an increase in [Ca]° and lengthened by a decrease in [Ca]° The rate of action of altered [Ca]° was found to be high, both on twitch amplitude and contracture height, the effects of a change in [Ca]° being seen in less than one second. The results can be satisfactorily explained by assuming that the site of action for these calcium effects is the cell membrane.