In incubation of rabbit platelets with ADF, adrenalin, serotonin and thrombin the level of hydroxyperoxides serotonin and thrombin the level of hydroxyperoxides serotonin and thrombin the level of hydroxyperoxides they show, determined with respect to malonic dialdehyde, rises simultaneously with the enhancement of aggregation activity. Still higher level of malonic dialdehyde can be found in platelets of the animals with thrombosis of pulmonary vessels. The studies performed demonstrate that accumulation of malonic dialdehyde in platelets involves an initial stage of the thromboti process development. This finding can be used in the diagnosis of early stages of intravascular thrombogenesis.