Abstract: Ginsenoside Rg1 is an active principle isolated from Panax ginseng CA Meyer. Primary neuronal culture was performed with different concentrations of Rg1. On the 14th day, the culture was changed to serum-free medium. On the 16th day, neurons were harvested and assayed under microscope and fluorescence microscope. The result showed that serum withdrawal from the medium induced apoptosis in primary cultured cortical neurons. Rg1 (1, 10 μmol·L^-1) was shown to inhibit apoptosis and protect neurons against injury. Membrane fluidity was measured using fluorescence spectrophotometer in five groups of cultured cortical neurons: control (complete medium), apoptosis (serum-free medium), Rg1 0.1 μmol·L^-1, Rg1 1μmol·L^-1 and Rg1 10μmol·L^-1. Neurons were isolated enzymatically and loaded with the fluorescent dye, DPH (1,6-diphenyl-1,3,5-hexatriene). Membrane fluidity in control neurons (η 1.2928±0.1426) was shown to be significantly higher than that in apoptosis cells (η 2.9277±0.2004). The membrane fluidity of neurons treated with Rg1 (0.1, 1, 10 μmol·L^-1) increased significantly (η 2.9172±0.1604, 2.4731±0.2187, 1.6436±0.1483). These findings indicate a substantial alteration of membrane fluidity with neuronal apoptosis. Increment of membrane fluidity provides an aspect of elucidating the mechanism of Rg1′s anti-apoptosis function.