Ca 2+, calmodulin-dependent myosin light chain phosphorylation is generally considered to be an important regulatory mechanism Of smooth muscle contraction. We investigated the length-dependence of myosin phosphorylation and active stress induced by K + depolarization in arterial smooth muscle by measuring the two variables in the swine carotid media held at three steady-state tissue lengths -optimal length for contraction (Lo), 1.5 Lo, and slack length. We found that the length dependence of peak and steady-state myosin phosphorylation with respect to tissue length was different. Peak myosin phosphorylation was highest at L o but lower at both slack length and 1.5 L o, whereas steadystate myosin phosphorylation was similar at both L o and 1.5 L o, but lower at slack length. Stretching tissues to 1.5 Lo did not significantly change the steady-state myosin phosphorylation induced by K + depolarization, but releasing tissues to slack length was associated with a 42% decrease in the steady-state myosin phosphorylation induced by K + depolarization. These data indicated that one or more steps coupling membrane depolarization and Ca2+-dependent myosin phosphorylation were length sensitive. Additional data from skinned tissue experiments indicated that the length-sensitive step was not the coupling between Ca 2+ and myosin phosphorylation. Therefore, these data together suggest that one or more steps coupling membrane depolarization and the increase in cytosolic Ca 2+ concentration are length sensitive.