The study was conducted to observe in rats the possible modification of ectopic calcification by magnesium-orthophosphate-fluoride combinations, used as additives of diet for reduction of the cariogenicity of the sucrose. In rats, fed low magnesium diets, extra dietary orthophosphate (2%) considerably elevated the calcification of kidneys. Further additions of magnesium and fluoride partially reduced this adverse effect of phosphate. While the calcium content of the aorta in rats, fed low magnesium-high phosphate diet, was considerably elevated, the further addition of magnesium (40 ppm) partially reduced the calcifying effect of phosphate in aorta. Fluoride (15 ppm) together with magnesium (40 ppm) completely reduced it. The appearance of renal calculi caused by a low magnesium diet or by extra phosphate were similar according to light and electron microscopy except for the larger size in the latter case and occasional extratubular calculi found in groups with high phosphate-low magnesium and high phosphate with added magnesium diets.