Cationic fluxes resulting from glutamate receptor activity have recently been implicated in neurotoxicity. Immature cortical neurons are insensitive to the toxic effects of glutamate receptor stimulation. However, these neurons are killed by glutamate via a non-receptor-mediated mechanism thought to stem from glutamate's ability to inhibit cystine uptake. To examine the basis for their resistance to receptor-mediated toxicity, we have studied the development of glutamate receptor-mediated inward currents in cortical neurons in culture using the whole-cell voltage-clamp technique. We report that in immature cortical neurons (prepared from day-17 fetal brain and cultured for 1-3 days), N-methyl-D-aspartate, quisqualate, and glutamate are able to evoke only very small inward currents in a low percentage of neurons. After 7 days of culture, greater than 80% of neurons examined exhibited currents activated by these glutamate receptor agonists. Although most neurons expressed glutamate agonist-evoked currents after 7 days in culture, the amplitude of these currents was less than 10% of that observed after 15 days in culture. In contrast to currents activated by glutamate receptor agonists, those activated by gamma-aminobutyric acid reached maximal levels after only 2 days of culture. These results indicate that the delayed development of glutamate receptor-mediated currents accounts for the resistance of immature cortical neurons to glutamate receptor-mediated toxicity.