The response of rat neocortical pyramidal neurons (layers II–III) in vitro to brief periods of anoxia is a reversible depolarization of 3.8 ± 1.01 mV (mean ± S.E.M.; n = 114), which is accompanied by a moderate decrease in input resistance and significant depression of evoked synaptic activity. This effect is mimicked by ouabain, and is partially attenuated by the excitatory amino acid (EAA) antagonist, kynurenic acid. The estimated reversal potential (V_rev) for the anoxic depolarization (AD) is between −35 and −40 mV; in the presence of TTX a V_rev of −65 mV is obtained. Although a partial failure of NA⁺ − K⁺ pump activity and release of EAAs may contribute to the generation of the AD, other processes are likely to be involved.