Arylaminopeptidase and glycosidase activities were studied by histochemical staining methods in human tooth and supporting tissue ontogeny in order to demonstrate the possible metabolic differences in the differentiated and undifferentiated cells of the formation, modelling and re‐modelling metabolism of the tissue involved. The enzyme activities were studied using various N‐L‐ aminoacyl‐2‐naphthylamines and 1‐ and 2‐naphthol glycosides and ‐esters as substrates. The arylaminopeptidase activity (EC 3.4.11) was observed (1) in the basal cell layer of oral epithelium in the stomodeum, (2) in the subjacent mesoderm, (3) in the outer and (4) inner enamel epithelium, (5) in the dental papilla, (6) in the dental lamina, and (7) in the primordium of permanent tooth. No arylaminopeptidase activity could be observed in ameloblasts. odontoblasts. enamel, dentin, stellate reticulum and pulp, and the inner enamel epithelium also lost its activity during the stage of apposition. The following glycosidases were observed in the same structures as the arylaminopeptidases: β‐N‐acetylglucosaminidase (EC 3.2.1.30), β‐glucosidase (EC 3.2.1.21), β‐galactosidase (EC3.2.1.23). β‐glucuronidase (EC 3.2.1.31), as well as arylsulphatase (EC 3.1.6.1) The whole thickness of oral epithelium revealed β‐galactosidase and arylsulphatase activity, not only the basal cell layer. These observations were‐ thought to give support to the suggestions that differeniation is a stage of highly active metabolism and thus at the stage of apposition the cells of the outer enamel epith