The developmental profiles of the gangliosides and those of the fatty acids and long-chain bases of the total ganglioside mixture of the brain of chicken were followed from the 10th day of incubation to the 63rd posthatching day. One O-acetylated polysialoganglioside that seems specific of the earlier embryonic stage and up to 21 alkali-stable components could be recognized by high-resolution two-dimensional TLC procedures and quantified by computer-assisted two-dimensional TLC densitometry. Besides a number of gangliosides identified by co-chromatography with reference standards, 10 were of unidentified structure, and within these 4 did not belong to the gangliotetrahexosyl series. Throughout embryonic life, the ceramide portion of gangliosides was found to contain the long-chain base species with 18 carbons. Those with 20 carbons, approximately 10% of the total, were found to be present only after hatching.