The effect of external calcium and of temperature on the contractile responses has been studied in voltage clamped snake twitch muscle fibers. Increasing [Ca(++)](o) from 0.2 to 7.0 mM raised contractile threshold by 15-20 mV, the latter coinciding with the appearance of delayed rectification. The duration of contracture, the rates of rise and decay of tension depended on the level of depolarization and [Ca(++)](o). The minimum duration of repolarization necessary to restore the contractile response was much shorter in high [Ca(++)](o). When the bathing solution was cooled to 10 from 20 degrees C the time-course of contracture was markedly prolonged and the outward current was reduced without significant change in maximum tension. The threshold for contraction tended to be somewhat lower at the lower temperature. The contractile repriming was much slower at low temperature. However, reduction in temperature slowed the rate of recovery much less at low [Ca(++)](o) than at normal [Ca(++)](o).