Fluoride can inhibit caries at plaque-retention sites, but some studies indicate that fluoride is less effective in fissures than on smooth surfaces. To study the efficacy of fluoridated toothpastes at plaque-retention sites, an intra-oral model was used with bovine coronal dentine discs, in which grooves of two different widths were sawn. The discs were mounted in the partial prostheses of 31 participants divided into two groups. One group brushed with a non-fluoridated toothpaste and a second with a paste containing 1,000 ppm fluoride as NaF After 3 months, the specimens were retrieved and from each a thin section was taken for microradiographic analysis. Lesions which developed in the grooves resembled natural lesions in terms of the presence of a surface layer and the mineral content profiles. Extensive lesions followed the direction of the dentinal tubules. The mineral loss was quantified half-way into and at the base of the grooves and ranged from 0 to 20,000 vol% × μm. Analysis of variance showed that the mineral loss was significantly influenced by the treatment and the width of the grooves (p < 0.001). In the broad grooves the average mineral loss was 19% smaller in the fluoride group than in the non-fluoride group, in the narrow grooves this value was 7%. Taking the two treatment groups together, the average mineral loss was largest half-way into the broad grooves (4,921 vol% × μm) and smallest at the base of the narrow grooves (2,289 vol% × μm). The results with this new intra-oral model indicate that the dimensions of small grooves in dentine, and thus their accessibility, determine not only their susceptibility to caries but also the protective effect of a fluoridated toothpaste.