The dynamics of actin polymerization, cytoskeleton formation, and interaction with membrane and cytoplasmic proteins as a result of platelet activation by temperature. ADP, or thrombin were studied. The polymerization of about 30% of platelet DNase I available actin to a nonavailable state is rapid and complete within 10 s after platelet activation with ADP and thrombin. This polymerization might be related to shape change rather than to aggregation or secretion. A similar value of actin polymerization is obtained when platelets are induced to change shape by cooling. This polymerization is partially reversible upon deactivation of the platelets by apyrase, hirudin, or rewarming. Cycles of temperatures-mediated activation and deactivation show a cyclic variation in the state of actin, with a tendency to refractivity to further changes after a couple of cycles. No correlation is observed between microtubule integrity and actin polmerization when studies are performed with platelets pretreated with colchicine. Analysis of the Triton residue composition shows that the cytoskeleton of resting platelets is composed mainly of actin and myosin in a 4.5:1 ratio. Activation with ADP and thrombin leads to the association and incorporation of several other protein (actin binding protein, 95 000 daltons, three to four proteins in the 35 000-dalton region, and two proteins in the 17 000-dalton region with the cytoskeleton). The incorporation of these proteins has a dynamic nature that depends on both the state of aggregation and the reversibility of the activation. Activation leads to a significant increase in the total cytoskeletal proteins, and although low temperature also induces such an increase, the cytoskeletal pattern of cooled platelets is not different from that of resting platelets. A complete reversibility in morphology and amount of protein was observed with temperature cycling. In light of these results, the dynamic nature of the state of actin in platelets is discussed.