The in vitro experiments for isometric contraction were done to investigate the different action mechanism of calcium ion on phasic and tonic components of K-contracture in guinea pig s taenia coli. The results were as follows: 1) The degrees of K-contracture were increased gradually from 4 mM to 15, 20, 40 & 100 mM of[K<sup>+</sup>]<sub>0</sub>. The maximal developed tension in tonic component was observed in 100 mM of [K<sup>+</sup>]<sub>0</sub>. 2) The phasic components were not so affected by 2, 4, 8 & 16 mM of [Ca<sup>++</sup>]<sub>0</sub> in 100 mM K-contracture, but the tonic components were gradually increased in a dose-responsive manner. 3) The K-contracture was not influenced by norepinephrine, 1 mg/l, but was completely abolished by verapamil, 2 mg/l. 4) The phasic component was little affected by verapamil, 0.01 mg/l, 0.1 mg/l, and 1 mg/l, but the tonic component was completely disappeared at the concentration of verapamil 1 mg/l. The above results suggest that [Ca<sup>++</sup>]<sub>0</sub>. primarily affected the tonic component than the phasic component of K-contracture in guinea pig s taenia coli.