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Keywords: smooth muscle myosin light chain kinase/autophosphorylation/chicken gizzard/MLCK ABSTRACT.Myosin light chain kinase (MLCK)is a calmodulin-dependent protein kinase which phosphorylates the 20,000 dalton regulatory light chain of myosin II. Here we show that activation of chicken gizzard MLCKby Ca2+/calmodulin is inhibited by autophosphorylation at 2 sites in the absence of Ca2+/calmodulin. Two phosphorylation sites are located in the functional domain of the kinase, the threonine site toward the actin binding domain near the N-terminus of MLCKand the serine site in immediate proximity to the calmodulin binding site. The autophosphorylation was significantly inhibited by the binding of calmodulin to MLCKin the presence of Ca2+.