The time course of interaction of caldesmon with actin may be monitored by fluorescence changes that occur upon the binding of 12-(N-methyl-N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl))-labeled caldesmon to actin or to acrylodan actin. The concentration dependence of the observed rate of caldesmon-actin binding was analyzed to a first approximation as a single-step reaction using a Monte Carlo simulation. The derived association and dissociation rates were 10(7) M-1 s-1 and 18.2 s-1, respectively. Smooth muscle tropomyosin enhances the binding of caldesmon to actin, and this was found to be due to a reduction in the rate of dissociation to 6.3 s-1. There is no evidence from this study for a different mechanism of binding in the presence of tropomyosin. The fluorescence changes that occurred with the binding of 12-(N-methyl-N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl))-labeled caldesmon to actin or actin-tropomyosin were reversed by the addition of myosin subfragment 1 as predicted by a competitive binding mechanism.