The kinetics of binding of the nonhydrolyzable nucleotides ADP and adenosine 5''-(.beta.,.gamma.-imidotriphosphate) (AMP-PNP) to [rabbit muscle] myosin subfragment 1 (SF-1) and actosubfragment 1 (acto-SF-1) were reinvestigated. The nucleotide binds in a rapid equilibrium step (K0), followed by 2 first-order fluorescence transitions with k1 + k-1 .mchgt. k2 + k-2. The rates and amplitudes of the fluorescence transitions are different for ADP and AMP-PNP and in turn can be distinguished from the corresponding steps involved in ATP binding. The similarity in the maximum rate of the observed fluorescence signal for ADP and ATP binding to SF-1 and 0.1 M KCl is fortuitous as the maximum rates differ greatly at higher ionic strength. Under favorable conditions of high ionic strength where the amplitude of the fluorescence enhancement is large, the binding of AMP-PNP to acto-SF-1 gave a fluorescence change prior to dissociation, followed by a second fluorescence transition at the same rate as the dissociation of the proteins. Thus a conformation change precedes the nucleotide-induced dissociation of actomyosin. At least 3 acto-SF-1-nucleotide complexes are necessary to explain the kinetic behavior.