Using hexokinase, glucose, and ATP to vary reversibly the concentrations of ADP and ATP in solution and bound to Acanthamoeba actin, I measured the relative critical concentrations and elongation rate constants for ATP-actin and ADP-actin in 50 mM KCl, 1 mM MgCl2, 1 mM EGTA, 0.1 mM nucleotide, 0.1 mM CaCl2, 10 mM imidazole, pH 7. By both steady-state and elongation rate methods, the critical concentrations are 0.1 microM for ATP-actin and 5 microM for ADP-actin. Consequently, a 5 microM solution of actin can be polymerized, depolymerized, and repolymerized by simply cycling from ATP to ADP and back to ATP. The critical concentrations differ, because the association rate constant is 10 times higher and the dissociation rate constant is five times lower for ATP-actin than ADP-actin. These results show that ATP-actin occupies both ends of actin filaments growing in ATP. The bound ATP must be split on internal subunits and the number of terminal subunits with bound ATP probably depends on the rate of growth.