The developmental appearance of gangliosides in mammalian brain has been studied almost exclusively at postnatal stages. These studies show the presence at birth of at least four major gangliosides, with GMI (nomenclature of Svennerholm, 1963) t h e major monosialoganglioside (Suzuki, 1965;Vanier et al., 1971;Merat and Dickerson, 1973). In recent studies on fetal brain from rats (Berra et al., 1978) and mice (Rosner, 1977), the concentration of GMI has been reported to be relatively high at the earliest developmental stages studied, and to decline as development proceeds. However, our studies of ganglioside metabolism in retina and brain of the fetal rat have led us to believe that the dominant ganglioside at early stages in development is more likely Gnn than GMl.