Hippocampal slices maintained in an oxygen-rich static interface chamber remained viable, as determined by the mitochondrial marker 2,3,5-triphenyltetrazolium chloride (TTC), for over 20 h in vitro. By contrast, slices exposed, after 1 h in vitro, to an anoxic environment for 25 min and then allowed to recover for 1-18 h, showed an initial slight decrease in TTC staining followed by a dramatic decrease at time points greater than 6.5 h after anoxia. These data are suggestive of delayed neuronal death. Furthermore, the decreases in TTC staining induced by anoxia could be prevented by conditions known to prevent cell death either in vitro or in vivo. For example, pretreatment of the slices with the N-methyl-D-aspartate antagonist 3-((RS)-2-carboxy-piperazin-4-yl)-propyl-1- phosphonic acid dose-dependently prevented the loss of TTC staining induced by 25 min anoxia. In addition, high-intensity TTC staining correlated with normal CA1 synaptic activity, even after more than 20 h in vitro, suggesting that TTC staining reflects functional neuronal activity. These data suggest that the use of TTC staining of in vitro hippocampal slices may represent a novel and convenient screen for anti-ischemic compounds.