Excitotoxic disturbances during brain development were studied in the mouse using intracerebral injections of ibotenate, a glutamatergic agonist of the N-methyl-D-aspartate (NMDA) complex receptor, to analyse the protective effect of a systemic bolus of MgSO4, a non-competitive antagonist of the NMDA ionophore-complex receptor. MgSO4 did not prevent microgyia, induced by ibotenate when injected at P0 immediately after the post-migratory settlement of layer V, but did prevent ulegyrias, porencephalic cysts, and other cortical and cortical-subcortical hypoxic-like lesions arising after completion of the neocortical cyto-architectonic development at P5. Protection was optimal in 80 per cent of mice at 600mg/kg, with no mortality due to MgSO4; thereafter mortality increased with dosage. The protective effect appears after the developmental acquisition of two properties of the excitotoxic cascade, namely the coupling of the massive calcium influx with NMDA-receptor overstimulation and the predominance of magnesium-obliterable calcium channels. This animal model supports the clinical intervention studies with magnesium in hypoxias/perfusion failures and has implications for their design. If maturation of the excitotoxic cascade follows the same sequence in humans, protection is probably low before 26 weeks of gestational age.