Two control diets with different sucrose contents (44% and 22%) were given to weanling rats for 55 d. The test diets were supplemented with an F + Mg combination, 75 parts/10(6) F as NaF + 50 parts/10(6) Mg as MgO and 38 parts/10(6) F + 25 parts/10(6) Mg, respectively, i.e. using the same supplement/sucrose ratio in each test diet. The supplementation in each test diet caused the same percentage fissure caries reduction, but the absolute mean caries scores were higher in the high-sucrose groups than in the respective low-sucrose groups. Enamel solubility was reduced by the supplementation. Despite the high fluoride supplementation, no untoward effects were recorded in growth and in aortic, heart, and renal calcium contents. Bone mineral density was reduced in the low-sucrose controls and slightly reduced in the high-sucrose F + Mg supplemented rats, as compared with the high-sucrose controls.