Rat incisors were fixed with a solution of 0.05% Cuprolinic Blue and 2.5% glutaraldehyde in the presence of various concentrations of MgCl2 according to the critical electrolyte concentration (CEC) principle. This method allows glycosaminoglycans (GAG) to be properly preserved and visualized. Small granules were stained by the cationic dye in the predentine in the absence of MgCl2. These granules grew in size and became more electron-dense when the concentration of the electrolyte was increased. Larger ribbon-like structures and granules were seen when 0.3 M MgCl2 was used. In the dentine, tiny dots in close association with the surface of the collagen fibres, or their periodic striations, were positively stained. A thick electron-dense band located on the dentine side at the predentine-dentine junction was seen both with and without 0.05 M MgCl2. With higher concentrations of the electrolyte (0.1-0.3 M), this band was reduced to a very thin line located at the border of the dentine, along with mineralizing collagen fibres. This demonstrated the presence of GAG at the dentine surface and therefore indicated that GAG may play a role as nucleator agent.