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1. The actions of Sr2+ on the smooth muscle of the guinea-pig taenia coli have been studied under constant-current and voltage-clamp conditions. 2. In replacing Ca2+, Sr2+ (2.5 mM) prolongs the duration of the action potential by slowing the rate of repolarization, but increases the steady-state membrane conductance. These changes occur in preparations in Krebs solution as well as in preparations in a Na+-free tetraethylammonium medium. 3. When [Sr2+]o is increased to 20 mM, the amplitude of the spike is increased, and the duration shortened. 4. Under voltage-clamp conditions, Sr2+ carries the early inward current (Ia). Increasing [Sr2+]o increases the maximum Ia and shifts its reversal potential (Ea) towards more positive voltages, without affecting the conductance, ga. 5. Sr2+ does not affect the steady-state inactivation of Ia, but markedly slows the rate of inactivation. 6. In all concentrations, Sr2+ enhances the late outward current (Ib, chiefly Ik), by increasing the conductance, gb. The degree of increase is greater in higher [Sr2+]o. 7. Ca2+ at concentrations of 1 or 2.5 mM added to Sr2+ solutions partially reverses the above effects, suggesting the presence of some competition for the same channel or binding sites or both. 8. The results indicate that in the taenia coli, there is a component of the K+-conductance system which is sensitive to [Sr2+]o and to [Ca2+]o. 9. They also suggest that the inactivation of the early inward current is an important mechanism underlying the repolarization of the action potential, and that IK probably has a delayed and gradual onset.