Abstract: The protective effects of l-3-n- butylphthalide(l-NBP) and d-3-n-butylphthalide(dNBP) on KCl(20 mmol·L^-1)- or NMDA(N-methyl-D-aspartate, 30 μmol·L^-1)induced cytotoxicity in primary cultured rat cortical neurons were studied. Intracellular LDH release, percentage of cell death and cellular morphological changes were used to evaluate the effect of drugs. l-NBP(1～100 μmol·L^-1) and d-NBP(1～100 μmol·L^-1), but not nimodipine (1～100 μmol·L^-1), were shown to dose-dependently inhibit LDH release induced by NMDA (30 μmol·L^-1) in cultured rat cortical neurons with IC₅₀ values of 4.89 μmol·L^-1 and 13.52 μmol·L^-1, respectively. The percent cell death was reduced with IC₅₀ values of 44.37 and 49.78 μmol·L^-1, and the cellular morphology improved. The effect seemed to be the same as that of equal concentration of NAME(N^G-nitro- L- arginine methyl ester). In addition, l-NBP(10 μmol·L^-1), d-NBP(10 μmol·L^-1) and nimodipine(10 μmol·L^-1) also produced significant inhibition on intracellular LDH release and decrease in percent cell death induced by KCl(20 mmol·L^-1) in cultured neurons. The potencies of l-NBP and d-NBP were similar to that of equal dose of nimodipine. These data suggest that l-NBP and d-NBP can remarkably protect cultured neurons against the damage induced by KCl and NMDA.