1. The effects of removing the external Ca ions on the spontaneous and evoked activity of the smooth muscle of the guinea-pig taenia coli were investigated with the double sucrose-gap method.2. In Ca-free Locke solution the membrane was depolarized, the membrane resistance became low, the spike amplitude became small and the mechanical response decreased. In most preparations the electrical and mechanical activity was abolished within 10 min, but in some preparations the electrical activity continued for more than 30 min.3. In Ca-free solution containing 0.1 mM-EGTA, the membrane was depolarized and the electrical and mechanical activity was abolished within 5 min in every preparation. When NaCl was replaced with sucrose, the effects of Ca removal on the spike activity and contraction appeared very slowly and the membrane potential and membrane resistance remained unchanged.4. When Ca was replaced with Mg (2 mM) the spike was blocked within 1 min without depolarization or reduction of the membrane resistance. In Na-deficient (sucrose) solution, the presence of Mg accelerated the disappearance of the spike caused by Ca removal.5. In Ca-free solution containing 0.5 mM-Mg, a spike-like activity was observed without accompanying mechanical response. This activity was blocked by increasing the Mg concentration above 2 mM. It was Na-dependent, since it was abolished by removing Na from the external solution, but it was not influenced by tetrodotoxin (2 x 10(-6) g/ml.).6. It was concluded that calcium has at least two functions, one as current carrier for the action potential and another as controller of the Na permeability of the membrane. It was also suggested that the Ca which is bound at the membrane may be utilized as a source of Ca ions to carry the current for the action potential.