Spontaneous electrical and mechanical activity of isolated rat portal vein was abolished by cooling to 10°C as indicated by sucrose gap recordings. Graded increases in [K<sup>+</sup>]<sub>o</sub> at this temperature caused only transient propagated activity after which the muscle stabilized at constant membrane potentials and contracture tensions. Complete relaxation occurred on return to normal [K<sup>+</sup>]<sub>o</sub>. An S-shaped relation was found between log [K<sup>+</sup>]₀ and contracture force; the mechanical threshold corresponded to about 9 mM K<sup>+</sup> and the maximal response was reached at 40-50 mM. Variations in [Ca<sup>2+</sup>]<sub>o</sub> from 0.4 to 12.5 mM did not appreciably affect the relative concentration-effect curve for K<sup>+</sup>. Thus a graded relation between depolarization and contractile force has been demonstrated in this normally spike-generating smooth muscle.