The cytoplasm of motile cells contains a dynamic system of filamentous protein polymers that endow the cell with elasticity permitting it to maintain its shape in the presence of mechanical forces encounteredin vivo. Part of this cytoskeleton is composed of filaments of polymerized actin. Remodeling of this network is required for cell motility and cytoplasmic restructuring, and the reversible polymerization of actin per se has been suggested to cause morphologic changes such as cell ruffling and pseudopd extension. Changes in the degree of polymerization of acting and in the association of actin filaments into supramolecular structures are often associated with cell activation. Such activation is initiated by extracellular signals that bind to receptors which are often coupled by G‐proteins to the production of intracellular second messangers. Cytoplasmic gel‐sol transitions therefore can occur by formation and dissolution of actin networks, mediated by a variety of actin‐binding proteins which are regulated by intracellular signalling molecules such as Ca2+and polyphosphoinositides.The effects of three actin binding proteins: profilin, gelsolin and ABP (Tilamin) on the polymerization of actin and the viscoelasticity of the resulting networks measuredin vitrosuggest possible roles of these proteinsin vivo. In particular, gelsolin, which activated by Ca2+to sever and cap actin filaments, and released from filament ends by PIP2, appears to be a likely candidate for regulation of gel‐sol transitions in response to cell activation. Recent results demonstrate that the hydrolysis of ATP that occurs following actin polymerization also influences the structure of the resulting filament. In addition being regulated by acting‐binding proteins, the viscoelasticity of actin networks is also affected by the presence of the other two classes of cytoplasmic protein polymers, microtubules and intermediate filaments.