Using a novel device capable of generating plaque in vivo on a natural enamel substrate, it has been possible to determine fluoride profiles from the saliva-plaque interface towards the enamel surface. Fluoride profiles in dental plaques tended to fall from the saliva-plaque interface towards the enamel. The device also offered the possibility of examining fluoride distributions after pre-treatment of the enamel with fluoride in vitro. Fluoride profiles were determined in plaque generated in vivo on enamel surfaces, which had been previously treated with a 900-ppm fluoride solution. The results showed the previously reported fall from the plaque surface, but in addition, a further rise towards the enamel surface was seen. The data imply that enamel loaded with fluoride can release some of this fluoride back into the plaque and may act as a fluoride reservoir.