The temporal changes in human platelet actin polymerization, cytoskeletal morphology, and protein content that occur during thrombin-induced platelet activation were investigated by analysis of Triton-extracted platelets. Measurement of the DNase inhibitory activity of control platelets immediately after adding an equal volume of 2% Triton X-100, 10 mM ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, and 0.1 M Tris, pH 7.4, showed that approximately 50% of the actin in unstimulated platelets was filamentous and unable to inhibit DNase-catalyzed hydrolysis of DNA. Activation of platelets with thrombin for 15 s caused the amount of actin in the filamentous form to increase to approximately 65%. Examination of the morphology and protein composition of these filamentous structures showed that the cytoskeletal structures from control platelets consisted of a random array of filaments which contained 14% of the total platelet myosin and 6% of the total actin-binding protein. In contrast, the cytoskeletal structures of thrombin-activated platelets appeared as cytoskeletal structures of individual platelets. The composition of these cytoskeletons varied depending on the time of thrombin activation. Those from platelets activated with thrombin for 15 to 30 s contained 90% of the platelet myosin and 20% of the platelets with thrombin before Triton addition resulted in a decreased association of myosin to 60% with no change in either the actin or actin-binding protein content of the cytoskeletal structures. Since these changes are rapid and precede serotonin secretion, it is suggested that they are involved in the physiological response of the platelet.