Ethanol tolerance and erythrocyte membrane lipids were studied in Sprague-Dawley rats at various times during and after chronic administration, by inhalation, of ethanol vapor. Tolerance increased during the three weeks treatment period and reverted to base line ten days after the treatment was stopped. Chronic ethanol treatment led to changes in the composition of membrane phospholipid fatty acids. These changes partially reverted after treatment ceased. At all times the changes in 16:0, 18:0, 18:1 and 18:2 were correlated with the degree of ethanol tolerance. Analysis of the effect of ethanol treatment (ip injections over a one week period) in three strains of mice showed that the changes of phospholipid fatty acids in erythrocyte membranes were related to whether the strain developed a tolerance to the hypnotic effect of ethanol (DBA, C 57), or not (Swiss). These results show that membrane phospholipid fatty acid composition and ethanol tolerance co-vary during chronic treatment. During the withdrawal period, ethanol sensitivity reverts to control values while the return of the fatty acids to the normal state is incomplete.