To evaluate chronic ethanol toxicity on erythrocyte membrane and preventive action of betaine as a methyl donor, 24 male Wistar albino rats were divided into three groups: control, ethanol and ethanol plus betaine group. Animals were fed 60 ml diet per day for two months. Rats in the ethanol group were fed ethanol 8 g/kg/day. The ethanol + betaine groups were fed ethanol plus betaine (0.5% w/v). After two months, all animals were killed. Malondialdehyde, (MDA) and sialic acid (SA) levels were determined in plasma samples. Osmotic fragility tests were performed on whole blood samples and erythrocyte membrane thiol contents were determined using membrane suspensions.