Abstract—Essential fatty acid deficiency was induced in mice after feeding a fatty acid deficient diet for 6 months. Activity of the (Na++ K+)‐ATPase in the total brain homogenates and in isolated synaptosomal plasma membranes was significantly higher (P< 0 05) in the deficient mice than the controls. Analysis of the acyl group composition of phosphoglycerides in brain as well as in the synaptosomal plasma membranes showed that mice fed the deficient diet had increased levels of 20:3(n‐9) and 22:3(n‐9) and decreased levels of 20:4(n‐6) and 22:4(n‐6). However, acyl group changes varied among individual phosphoglycerides and were most obvious in the two species of ethanolamine phosphoglycerides. A decrease in 22:6(n‐3) level was also observed in some phosphoglycerides of the synaptosomal plasma membranes especially the diacyl‐sn‐glycerophosphorylserine. In this experiment, a new solvent system for chromatographic separation of the diacyl‐sn‐glycerophosphorylserine and diacyl‐sn‐glycerophosphorylinositol was reported. The separation technique was suitable for analysis of acyl group composition of individual phosphoglycerides by gas‐liquid chromatography. The results were consislent with a positive correlation of the non‐polar acyl groups of brain membranes with the active ion transport activity. The increase in enzymic activity during deficient state may be the result of a biological adaptation due to structural alte