The characterization of Ca~(2 )-independent phosphorylation of myosin by MLCK was investigated.The results indicated that MLCK phosphorylated myosin not only in the Ca~(2 )-dependent way,but also in a Ca~(2 )-independent way.By using MLCK,the Ca~(2 )-independent phosphorylation of myosin needed much higher concentration of MLCK,and was less influenced by increasing the incubation temperature,prolonging incubation time,and increasing the K~ -ion strength compared to the Ca~(2 )-dependcnt phosphorylation.The results suggest that the Ca~(2 )-independent phosphorylation of myosin by MLCK be a new mechanism and involve in maintaining the tension of smooth muscle.