The relationship between undernutrition-induced alterations in some myelin membrane-bound enzyme activities and phospholipid fatty acid composition of this membrane was ascertained in developing rat brains. Undernutrition was imposed in pregnant dams through gestation and lactation, (last 10 days of gestation, and through lactation) by feed restriction. Experimental groups of animals received 50% of the amount of diet consumed by controls. Pups born to these mother rats were killed at day 7, 14 or 21 of postnatal age. Myelin membrane was isolated from the major regions of the brain, and analysed for phospholipid fatty acid profiles, phosphoinositides and ganglioside species. While there were no diet-related differences in the activities of 5'-nucleotidase (EC 3.1.3.5), myelin phospholipids from cerebella and brain stems of experimental rats exhibited lowered proportions of the long-chain polyunsaturated fatty acids, C20:4 (n - 6) and C22:6 (n - 3) concomitant with elevated activities of ouabain-sensitive Na+/K+-ATPase (EC 3.6.1.4). Levels of diphosphoinositide, triphosphoinositide and trisialogangliosides also decreased in myelin from brains of experimental animals. These results suggest a relationship between myelin phospholipid fatty acid profiles as indicators of membrane unsaturation, and the possibility of allosteric modification of Na+/K+-ATPase activity.