The present work was undertaken to show the effect of age upon the level of calcium and inorganic phosphate in the blood of cattle of various ages, and to establish figures which could be regarded as the normal levels of these elements in the blood of healthy Canadian cattle. Thus it may serve as a basis for comparison when applying blood analyses to the investigation of disorders of cattle in Canada. The physiological variations in organic phosphate, as shown by Palmer et al (90) and in serum calcium found be Frei, and Emerson (30) and which will be discussed later, were in as far as possible avoided. The majority of the specimens were obtained at a local slaughterhouse. Animals known to have come from those localities with herbage suspected of being phosphorus deficient were not studied. The samples were obtained from animals drawn from many sections of southern Ontario, and form a representative group of the cattle therein. The cattle were all in apparent good health at the time of slaughter when the sample was obtained, and subsequently the carcase in each case passed inspection as being free from disease. The ages of the animals were determined by a careful examination of the subjects after death. Calves and cattle from the herd of the Ontario Agricultural College at Guelph were used as a control group. The ages of these animals could be accurately determined from the herd-book, and they were free from any possible alterations in the blood constituents resulting from the vicissitudes of shipment from distant parts, and of stockyard management. No analyses were made upon the blood of calves less than three weeks of age as younger animals were not to be found in the slaughterhouse pens. The cattle from which this blood was taken were practically all cross-breeds. But it has been shown by Malan, Green, and duTToit (66) that in cattle, breed makes no apparent difference to the calcium and inorganic phosphate in the blood. Therefore no attempt was made to record the breed of the subjects. Many workers when publishing their findings of blood inorganic phosphate and serum calcium record and discuss the two together. This is because of a primary interest in the diseases of man resulting from an abnormal metabolism of one of these substances, which because of their close interrelationship, usually affects the other. However, in this article they will be recorded separately. It has been noted that veterinarians (and the viewpoint of this paper has been that of the veterinarian throughout) are chiefly interested in diseases affecting only one or other of these two substances, for example calcium is the only blood constituent considered to show diagnostic fluctuations in "milk fever", and in aphosphorosis in cattle the lowering of the blood phosphate is considered to be the only material alteration to be found on blood analysis. No recent survey of the various physiological factors influencing the calcium and inorganic phosphorus of cattle blood, and of the effect they exert, has been found. Therefore some reference is given in the text to the findings of other workers in this field. Where a factor influencing the investigated constituents in cattle blood is well authenticated only those references pertaining to cattle are given. Where there has been some dispute over its influence collaborative references from similar work in man have been given. Where no reference can be found as to the effect of a certain physiological factor, known to have such effect in other animals, upon the constituents of bovine blood, the findings, if any, in man and other animals are noted.