Thrombin has been shown to play a major role in brain injury after intracerebral hemorrhage (ICH). In this study, we measured thrombin activity in the perihematomal zone and examined the role of thrombin in ICH-induced brain tissue loss. There were 2 experiments in this study. In the first part, adult male Sprague-Dawley rats received 100 microL of either autologous whole blood or saline. The rats were killed at 1 h or 24 h later for thrombin activity measurement. Thrombin activity was measured using the thrombin-specific chromogenic substrate, S2238. In the second part, rats received a 50-microL intracaudate injection of either thrombin or saline, and the rats were killed at days 1, 3, or 28 for determination of neuronal death and brain tissue loss. We found that brain thrombin activity was elevated in ipsilateral basal ganglia 1 h after ICH. Intracerebral injection of thrombin rather than saline caused significant neuronal death at days 1 and 3, and resulted in significant brain tissue loss at day 28. These results suggest that thrombin inhibition in the acute phase may reduce ICH-induced brain damage.