F-actin is the major component of muscle thin filaments and, more generally, of the microfilaments of the dynamic, multifunctional cytoskeletal systems of nonmuscle eukaryotic cells. Polymeric F-actin is formed by reversible noncovalent self-association of monomeric G-actin. To understand the dynamics of microfilament systems in cells, the dynamics of polymerization of pure actin must be understood. The following model has emerged from recent work. During the polymerization process, adenosine 5'-triphosphate (ATP) that is bound to G-actin is hydrolyzed to adenosine 5'-diphosphate (ADP) that is bound to F-actin. The hydrolysis reaction occurs on the F-actin subsequent to the polymerization reaction in two steps: cleavage of ATP followed by the slower release of inorganic phosphate (Pi). As a result, at high rates of filament growth a transient cap of ATP-actin subunits exists at the ends of elongating filaments, and at steady state a stabilizing cap of ADP.Pi-actin subunits exists at the barbed ends of filaments. Cleavage of ATP results in a highly stable filament with bound ADP.Pi, and release of Pi destabilizes the filament. Thus these two steps of the hydrolytic reaction provide potential mechanisms for regulating the monomer-polymer transition.