When chicks were fed excessive CaCo3, some of them manifested the toes and legs bent in an abnormal direction and final inability to walk in proportion to the dose of calcium. However, chicks fed CaHPO4 containing the same dose of Ca, instead of CaCO3, showed no abnormality of the legs. The leg abnormality of chicks fed excessive CaCO3 resembled the symptom of chicks fed a vitamin D-deficient diet, but there slight differences in serum Ca level and in the findings of the legs by roentgenography between both chick groups. In the radiograph of chicks fed excessive CaCO3, definite radiolucent area were found in all the bone joints related with poor mineralization. Furthermore, a slight elevation of Ca concentration, a decrease in inorganic phosphorous concentration and a remarkable increase in alkaline phosphatase activity were found in the sera of the chicks fed excessive CaCO3. In these chicks were observed the hypertrophic cartilage at the end of bones and an increase in osteoid seams at the diaphysis in the non-decalcified bone section. The findings described above suggested that an excessive Ca supply to chicks might cause a disturbance in P metabolism and produce hypophosphatemic rickets.