The fate of sulphated organic compounds, probably glycosaminoglycans (GAG), during amelogenesis was studied by autoradiography in molar teeth of guinea pigs that had received a single dose of 2.5 μCi/g of body weight of [35S]-sodium sulphate and killed from 10 min to 168 hr after injection. The sulphation site was the Golgi region of ameloblasts, from where the sulphated compounds migrated into the young enamel matrix along the entire zone lined by secretory ameloblasts. After forming an ill-defined band along the Tomes processes and the adjacent matrix, the 35S diffused through the whole thickness of the matrix, reaching the dentine-enamel junction. By correlating the silver grain concentration over matrix with the growth of the tooth. it was shown that the radioactive reaction, after attaining a maximum of intensity at 24 hr. decreased at 48 hr when the matrix moved from one region to the next, which was still related to the secretory ameloblasts. 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