Knowledge of the molecular mechanisms involved in chemomechanical transduction and the regulatory elements determining contraction and relaxation are largely derived from cross-striated muscle. However, it has become clear that vertebrate smooth muscles have features lacking in striated muscle. These derive from regulated crossbridge cycling rates observed as variable shortening velocities and ATP consumption rates. The special properties of smooth muscle are associated with differences in the physiological roles of muscle in the walls of hollow organs and reflect a common molecular motor governed by different regulatory mechanisms. A remarkably simple scheme involving Ca(2+)-dependent phosphorylation of crossbridges in smooth muscle can predict much of the mechanical and energetics behavior characterizing the muscle of hollow organs. Nevertheless, many unresolved issues are identified that are the focus of current research efforts.