Summary During the first 6 to 10 weeks of postnatal development of 11 normal calves, determination of hematocrit values and of hemoglobin levels in the blood and of the concentrations of several inorganic constituents in the blood and in the serum were made. Hematocrit values and hemoglobin concentrations declined during the first 3 weeks after birth but gradually increased during the following 7 weeks. Inorganic phosphorus of the blood serum increased from 6.7mg./100ml. at birth to 8.5mg./100ml. at 3 weeks, remaining relatively constant thereafter. Calcium of the whole blood ranged from 8.5mg./100ml. to 9.4mg./100ml. but revealed no consistent trends, whereas in serum the level decreased slightly but steadily from 12.3mg./100ml. during the period of colostrum consumption to 10.6mg./100ml. at 5 weeks of age. Magnesium of whole blood varied from 2.5mg./100ml. to 2.8mg./100ml., and of serum from 2.4mg./100ml. to 2.6mg./100ml., but the concentration in neither exhibited a definite trend as the calves developed. Concentrations of sodium in the whole blood decreased slightly the first 2 weeks but increased to the initial level within the following 3 weeks, the extremes being 276mg./100ml. and 313mg./100ml.; similar general trends were noted in the serum in which the range in concentrations was 346mg./100ml. to 384mg./100ml. Potassium of the whole blood declined steadily from an average of 141mg./100ml. at birth to an average of 53mg./100ml., the level in adult cattle, at 10 weeks; the concentrations in the serum exhibited little variation, ranging from 21mg./100ml. to 25mg./100ml. A comparison of the levels of minerals in whole blood and in serum indicated that the concentrations of sodium and calcium were greater in serum than in the cells; whereas the reverse was true of potassium and magnesium. Computations, based on hematocrit values, indicated that the amount of potassium in the cells predominated over that of sodium during the first 4 weeks of postnatal life; subsequently sodium concentrations predominated over potassium in the cellular fraction. This investigation was supported in part by a grant from Moorman Mfg. Co., Quincy, Illinois. The authors wish to express their appreciation to Mrs. Helen Hamlin and Mr. Roy Coleman for technical assistance in the analytical work.