The present study examined the appearance of thrombin activity in vitro after single and repeated in vivo balloon injury of the rabbit aorta. The in vitro ability of the injured vessel wall to bind and subsequently inhibit thrombin in the presence of defibrinogenated plasma was also assessed. Thrombin activity was assayed by measuring the levels of fibrinopeptide A generated in the presence of fibrinogen. These findings were correlated with the changes observed in light and electron microscopy. Thrombin activity on the vessel wall was increased five minutes and three hours after the initial and the repeated injury, and returned to control values one week after the initial injury. When the inhibition of thrombin was assayed in the presence of defibrinogenated plasma, a diminished inhibition capacity was observed after the repeated injury, which correlated with deposition of fibrin and an enhanced inflammatory reaction as measured by the density of granulocytes covering the injured neointima. Decreased thrombin inhibitory capacity of the injured neointima appears to be linked with its increased thrombogenicity.