In the rat sphincter pupillae, as in other smooth muscles, the primary signal transduction cascade for agonist activation is receptor  G protein  phospholipase C  inositol trisphosphate  intracellular Ca²⁺ concentration ([Ca²⁺]_i)  calmodulin  myosin light chain kinase  phosphorylated myosin  force development. Light stimulation of isolated sphincters pupillae can be very precisely controlled, and precise reproducible photomechanical responses (PMRs) result. This precision makes the PMR ideal for testing models of regulation of smooth muscle myosin phosphorylation. We measured force and [Ca²⁺]_i concurrently in sphincter pupillae following stimulation by light flashes of varying duration and intensity. We sampled at unusually short (0.01-0.02 s) intervals to adequately test a PMR model based on the myosin phosphorylation cascade. We found, surprisingly, contrary to the behavior of intestinal muscle and predictions of the phosphorylation model, that during PMRs force begins to decay while [Ca²⁺]_i is still rising. We conclude that control of contraction in the sphincter pupillae probably involves an inhibitory process as well as activation by [Ca²⁺]_i.