Effects of age and neonatal undernutrition were studied on total and fractions of gangliosides (GT1, GD1b, GD1a and GM1) in rat brain. GT1, GD1b + GD1a and GM1 are being presented here as polysialo-, disialo-, and monosialo-gangliosides. Undernutrition was induced by feeding mothers a low protein diet during lactation. The concentration of gangliosides increased to its maximum level by the age of 3 weeks and then decreased to its adult value by the age of 8 weeks. Polysialo gangliosides (GT1) comprised the maximum amount of gangliosides at birth (40%) and decreased to an adult value by 3 weeks of age. Disialo ganglioside GD1b increased to its maximum by 2 weeks and then leveled off. Disialo ganglioside GD1a and GM1 showed a decrease by 2 weeks and then reached to its adult value by 4 weeks. Neonatal undernutrition results in a decreased body and brain weight at all ages studied (7, 14 and 21 days) as well as total ganglioside concentration by 36%, 15% and 55% respectively at 7, 14 and 21 days. Polysialo gangliosides (GT1) constitute the major fraction of gangliosides at 21 days in undernourished rats as compared to controls, whereas mono- and di-sialo gangliosides remained decreased at all three ages compared to controls. This indicates the possibility of an underlying metabolic defect in undernourished animals.