The demonstration of a precursor-product relationship in the course of GM1 and GD1a biosynthesis is described in the present paper. We injected rats with GM2 gangliosides [GalNAc beta 1----4(NeuAc alpha 2----3)Gal beta 1----4Glc beta 1----1'Cer] of brain origin, which were isotopically radiolabeled on the GalNAc ([GalNAc-3H]GM2) or sphingosine ([Sph-3H]GM2) residue. We then compared the time-courses of GM1 and GD1a biosynthesis in the liver after the administration of each radiolabeled GM2 derivative. After the administration of [GalNAc-3H]GM2, GM1, and GD1a were both present as doublets, that could be easily resolved on TLC. The lower spot of each doublet was identified as a species having the typical rat brain ceramide moiety and represented gangliosides formed through direct glycosylation of the injected GM2. The upper spot of each doublet was identified as a species having the typical rat liver ceramide moiety and represented gangliosides formed through recycling of the [3H]GalNAc residue, released during ganglioside catabolism. After the administration of [Sph-3H]GM2, only ganglioside with the rat brain ceramide moiety were found, that represented the sum of ganglioside formed through direct glycosylation and those formed through recycling of some sphingosine-containing fragments. In each case, the time-course of GM1 and GD1a biosynthesis exhibited a precursor-product relationship. The curve obtained from the direct glycosylation showed a timing delay with respect to those obtained from recycling of GM2 fragments. These results are consistent with the hypothesis that the sequential addition of activated sugars to a sphingolipid precursor is a dissociative process, catalyzed by physically independent enzymatic activities.