Increasing the amount of iron by means of stomach tubing FeSO4 to rats significantly increases liver Fe content. When 50 mg phosphate is given silultaneously with Fe, a significant reduction in liver Fe content results, as well as a significant diminution in C and P content of newly forming incisor dentin. These results compared with a significant reduction in blood P concentration, suggest that increasing the Fe content of the diet prolonged periods of time without significantly increasing the dietary P content may result in improperly calcified tissues.