Objective: To investigate the effect of intracellular free Ca 2+ concentration ([Ca 2+ ] i ) on injury following transient Mg 2+ free treatment in vitro in developing cortical neurons. Methods: Embryo cortical neurons of rats cultured for 6 d and 17 d were directly exposed to Mg 2+ free media, or pretreated with NMDA receptor antagonists or calcium channel antagonist before being exposed to Mg 2+ free media. MTT assay was used to study the injury of neurons. [Ca 2+ ] i were measured using fluo 3, a fluorescent calcium sensitive dye and laser scanning confocal microscope, and calculated by the fluorescent intensity. Results:Compared with control, MTT conversion rates decreased after transient (3 h) Mg 2+ free treatment in neurons cultured for 6 d and 17 d in vitro, (59.1?6.87)% and (51.2?5.90)%, respectively. In neurons pre and co treated with 10 ?mol?L -1 MK 801, 50 ?mol?L -1 AP 5 and 10 ?mol?L -1 nifedipine, MTT conversion rates were higher than those of neurons with only Mg 2+ free treatment. Peak values of [Ca 2+ ] i in neurons cultured for 6 d and 17 d were 2.4?0.23 and 3.2?0.32, respectively Peak value of neurons 17 d in vitro was significantly higher than that of neurons 6 d in vitro (P