An in-depth understanding of the regulation of contractile tone in smooth muscle takes its form from detailed biochemical descriptions of (a) specific effects unique to individual effector inputs at the membrane,(b) mechanisms of mediation of the effector signal (second messengers), and (c) modes and sites of action of second messengers both within the contractile protein regulatory system and within the membrane-associated homeostatic mechanisms. Most early physiological and pharmacological studies on smooth muscle prepara tions focused primarily on extracellular effectors and their interactions with the membrane components ultimately involved in the control of intracellular Ca2+ concentration. Evaluation of the steady-state contractile response has been interpreted as an index of sarcoplasmic Ca2+ concentration, with the under lying assumption that Ca2+ activates contractile proteins through a simple switch-like mechanism. The discoveries that the 20, OOO-dalton light chain of smooth muscle myosin could be phosphorylated (1), and that the phosphoryla tion of smooth muscle myosin is associated with an increase in its actin activated Mg2+-ATPase activity (2), have led to a number of biochemical investigations into the regulatory role of the specific phosphorylation of myosin by the Ca2+-and calmodulin-dependent enzyme myosin light chain kinase. Recent investigations have focused on demonstrating the physiological role of myosin phosphorylation in the regulation of contractility in intact smooth muscle.