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Twelve Osborne-Mendel rats were given, for sixty days, an anticariogenic diet where 4% of the sucrose (2.7% of the diet) was replaced by an alkali phosphate salt combination (Na2HPO4+NaH2PO4·H2O+KH2PO4; mole ratios 4.65/0.52/1.00 respectively). Nephrocalcinosis occurred in every animal as small concentric calcium deposits in the medulla and as large calcified masses higher in the cortex. A slight peritubular inflammatory reaction occurred and many exfoliated cells were seen in the lumina of the collecting ducts. In the electron microscope, calcified masses seemed to erode the tubular epithelium. No mitochondrial calcification in the epithelial cells, or calcification in the tubular basement membrane, were found. The cytosomes in a few proximal tubules displayed dark condensations. The ultrastructural features were similar to those found in connection with magnesium deficiency. No calcification was found in 7 controls or in 7 rats receiving the same basic diet with a 4% bicarbonate-phosphate supplement in the sucrose for four months. The appearance of nephrocalcinosis synchronously with a caries-protecting effect and apposition of dental calculus in animals fed on diet supplemented with alkali phosphate is discussed.