Actin, one of the most highly conserved and abundant eukaryotic proteins, is constantly being polymerized and depolymerized within cells as part of cellular motility, tissue formation and repair, and embryonic development. Many proteins exist that bind to monomeric or filamentous (F) forms of actin to regulate the polymerization state. It has become increasingly apparent that the ability of different proteins to bind to and regulate actin filament dynamics depends on the ability of the filament to exist in altered conformations. Yet, little is known about how these conformational changes occur at the molecular level. We have destabilized F-actin filaments by forming a disulfide that locks the "hydrophobic plug" to the body of the actin subunit or by altering the C terminus of actin with a tetramethylrhodamine label. We also examined F-actin filaments at short times after the initiation of polymerization. In all three cases, a substantial fraction of protomers can be found in a "tilted" state that also is induced by actin depolymerizing factor/cofilin proteins. These observations suggest that F-actin filaments are annealed over time into a stable filament and that actin-depolymerizing proteins can effect a time reversal of polymerization.