Abstract Patterns of mineralization in the hard tissues of the roots of dog premolar teeth were investigated by administering tetracycline hydrochloride at various stages of root development. Nearer the crown, no continuous labelling was detected along the root surface, indicating that no cementum formed there up until the time of root completion. In the mid-root region, surface labelling was observed but interpreted as delayed mineralization of dentine rather than apposition of cementum. In the apical region, the patterns again suggested a delayed mineralization of the dentine surface prior to the apposition of cellular cementum. The labelling in this region also indicated that cementum formation spread from the apical region towards the crown.