Small strips of intestinal or arterial smooth muscle composed of many functionally skinned muscle cells (nonfunctional sarcolemma), skinned skeletal single fibers, or bundles of skinned cardiac fibers were used to test the hypothesis that a myosin light chain kinase/phosphatase system is responsible for the activation of contraction. The results showed that in smooth muscle: 1) there is a close correlation between the degree of phosphorylation of the myosin light chains and tension development: 2) irreversible thiophosphorylation of myosin light chain results in irreversible activation of tension; and 3) inhibition of the light chain kinase by phenthiazines results in dephosphorylation of the myosin light chains and inactivation of tension in the presence of Ca2+. In contrast, in skinned striated muscle fibers 1) there is no correlation between phosphorylation and tension, 2) light chains could not be thiophosphorylated, and 3) phenothiazines did not affect Ca2+-activated tension. These findings indicate a Ca2+-sensitive light chain kinase/phosphatase system is responsible for the activation of smooth muscle, but no similar evidence was found for such a system in striated muscle.