Chick blastoderms were cultured for 2 h in the presence of35S-sulphate. The distribution of the grains after light microscope autoradiography was compared in blastoderms during the elongation and during the shortening of the primitive streak. A uniform labeling was observed over the cells in both groups. Accumulation of grains was present in both groups at the ventral side of the upper layer, where transmission electron microscope studies have revealed a basal lamina. An additional accumulation of grains occurred over the cells and in the extracellular spaces of the head process and of the rostral part of blastoderms with shortening primitive streaks. This positivity could be correlated with the presence of ingressing and recently ingressed notochordal cells. Treatment of the sections with chondroitinase ABC and/or HNO2 before dipping in the nuclear emulsion demonstrated that at least chondroitin sulphate and N-sulphated heparan sulphate were present.