(1) It has been shown that the renal papilla of the hamster, the dog and the rat contain a high concentration of calcium and that there is a concentration gradient between the papillary calcium and that of the medulla and cortex. (2) The calcium and sodium gradient between renal papilla and medulla are parallel. (3) There is a gradient between the calcium concentration in the urine and that in the papilla in the rat and dog. This is not so in the hamster but a high proportion of calcium in the hamster urine may be in the form of collodial particles. (4) In the hamster, the calcium concentrated in the papilla can be lowered by a low calcium intake but this does not lower the sodium concentration in the papilla. (5) It is suggested that the high concentration of calcium in the renal papilla is related to the formation of Randall’s plaques and may be a factor in the formation of renal stones.