Abstract: Membrane fluidity was measured using fluorescence spectrophotometer in cortical cells isolated from Wistar rats of five age groups (fetal); neonatal (3 days), young (3 months), adult (9 months) and old (27 months). Neurons were enzymatically isolated and loaded with the fluorescent dye, DPH (1,6-diphenyl-1,3,5-hexatriene). The membrane fluidity of neonatal cells was shown to be significantly higher (η1.485±0.211) than that in young cells (η2.220±0.169), and that in young cells was significantly higher than that in old cells (η2.842±0.143). No significant difference in fluidity, neither between fetal and neonatal cortical cells nor between young and adult ones was observed. Ginsenoside Rg1 (Rg1) is one of the important active principles of ginseng and shares many pharmacological effects of this plant. When treated with Rg1 (10, 20, 40 mg ·kg^-1), the membrane fluidity of old cortical cells significantly increased (η2.670±0.108, 2.381±0.123, 2.000±0.101). These findings indicate a substantial alteration of membrane fluidity with neuronal aging. Increment of membrane fluidity provides an aspect in elucidating the mechanisms of Rg1's antiaging action.