Appendix Fig. Fluoro-Gold stained the enamel yellow within a few minutes after being placed into the sulcus on the mesial aspect of the first maxillary molars of this young adult rat. It also dissolved in sulcular fluid, moved around the first molar sulcus to reach the second molar, and spread into intercuspal grooves. The most intensely yellow regions corresponded to the areas with greatest influx of Fluoro-Gold into the pulp, shown in the text in Figs. 1-3.