The acute neurotoxicity produced by glutamate and related excitatory amino acids is probably caused by depolarization leading to excessive anionic and cationic fluxes and osmotic lysis. Recently, a more delayed form of glutamate neurotoxicity, which is critically dependent upon calcium influx, has been described in cultured neocortex. We investigated this phenomenon in cultures of dispersed rat hippocampal neurons. When these cultures were briefly incubated with various excitatory amino acids in low extracellular chloride, there was no acute toxicity, but a gradual drop-out of neurons occurred over the next day. When calcium was removed from the extracellular medium during amino acid incubation, this late neuronal loss was not seen. Interestingly, blocking excitatory amino acid receptors in cultures after the amino acid exposure also prevented this delayed neuronal death. In addition, these treated cultures contained neurons with normal physiological properties, and had concentrations of adenosine triphosphate that were close to control values. The findings suggest an amino acid-induced calcium influx may elevate the release of endogenous excitatory transmitter, likely glutamate, and/or increase the sensitivity of these neurons to glutamate. These in vitro observations may partially explain the delayed neuronal loss seen in some pathological conditions affecting man.