Thrombin can perform either protective or deleterious effects in a concentration-dependent manner. Organotypic hippocampal slice cultures were exposed to OGD and/or thrombin as indicated. The presence or absence of thrombin is indicated by (+) and (−), respectively. At 50 pM (0.01 unit/ml), thrombin caused protection against experimental ischemia, whereas at 500 nM (100 units/ml), thrombin was cytotoxic per se. Note that even 50 nM (10 units/ml) thrombin was deleterious when applied in combination with OGD. Intermediate thrombin concentrations of 500 pM–5 nM (0.1–1 unit/ml) did not affect the survival of the slice cultures. For each experiment, the mean ± SEM with n between 3 and 22 is shown. *, P < 0.025; **, P < 0.001.