Global ischemia elicits marked, transient caspase-3 activity in preconditioned neurons. Representative brain sections at the level of the dorsal hippocampus from rats subjected to sham operation (control, a), preconditioning (b–e), global ischemia (f–i), or preconditioning, followed by global ischemia (j–m). Sections were labeled with FAM-DEVD-FMK, a fluorescein-tagged analog of zDEVD-FMK, which binds irreversibly to activated caspase-3. Boxed regions are shown at higher magnitication in insets. a, In sections of control brain, caspase-3 activity was undetectable. b–e, Preconditioning induced a substantial, but sublethal, increase in caspase-3 activity in CA1 neurons, evident from 12–48 hr after preconditioning. f–i, Global ischemia induced pronounced caspase-3 activity in the CA1 pyramidal cell layer, evident from 6–48 hr after ischemia. j–m, In animals subjected to preconditioning followed by global ischemia, caspase-3 activity was robust at 6 hr, but declined sharply by 12 and remained low. n, Summary of data for animals subjected to preconditioning (light gray) or to preconditioning followed by global ischemia (dark gray). o, Summary of data for animals subjected to global ischemia only. Statistical significance was assessed by ANOVA, followed by Newman–Keuls test (**p < 0.01). Scale bars: 400 μm (low magnification); 50 μm (high magnification).