Preconditioning with sublethal ischemia induces tolerance to subsequent lethal ischemia in neurons. We investigated electrophysiologic aspects of the ischemic tolerance phenomenon in the gerbil hippocampus. Gerbils were subjected to 2 minutes of forebrain ischemia (preconditioning ischemia). Some of them were subjected to a subsequent 5 minutes of forebrain ischemia 2 to 3 days after the preconditioning ischemia (double ischemia). Hippocampal slices were prepared from these gerbils subjected to the preconditioning or double ischemia, and field excitatory postsynaptic potentials were recorded from CA1 pyramidal neurons. Capacity for long-term potentiation triggered by tetanic stimulation (tetanic LTP) was transiently inhibited 1 to 2 days after the double ischemia but then recovered. Latency of anoxic depolarization was not significantly different between slices from preconditioned gerbils and those from sham-operated gerbils when these slices were subjected to in vitro anoxia. Postanoxic potentiation of N-methyl-D-aspartate (NMDA) receptor-mediated transmission (anoxic LTP) was inhibited in slices from gerbils 2 to 3 days after the preconditioning ischemia, whereas it was observed in slices from sham-operated gerbils and gerbils 9 days after the preconditioning ischemia. These results suggest that protection by induced tolerance is (1) not only morphologic but also functional, and (2) expressed in inhibiting postischemic overactivation of NMDA receptor-mediated synaptic responses.