Short-term and long-term effects of ethanol on the fatty acid composition of mitochondrial, microsomal and myelin fractions in brain have been investigated. Microsomal membranes were not modified by treatment for 60 hr, while in mitochondrial membranes there was a significant decrease in arachidonic and docosahexaenoic acids, responsible for the decrease in the double-bond index. A clear decrease in the 18:1/18:0 ratio was found in myelin after short-term treatment, whereas no significant variations were observed in the other subcellular membranes under the same conditions. On the other hand, chronic exposure to ethanol for 18 days induced a significant increase in oleic and docosahexaenoic acids in microsomal membranes. However, no significant changes were detected in the composition of fatty acids of mitochondrial membranes after 18 days of administration of ethanol. Contrary to that found with short-term treatment, a significant increase was observed in the 18:1/18:0 ratio of the myelin fraction after chronic consumption of ethanol. These results suggest that alcohol intoxication of neonatal chicks induces different modifications in composition of fatty acids of different membranes in the brain, those observed in the myelin fraction being specially important.