Chronic kidney disease (CKD) affects one out of three cats twelve years or older. Calcium and phosphorus have major roles in kidney function. Like other animals, cats have a dietary requirement for calcium and phosphorus, and these nutrients must be provided in an appropriate ratio (approximately two parts calcium for one part phosphorus). Excessive intake and an unbalanced ratio are associated with progression of CKD in affected cats. As such, routine assessment of dietary and blood phosphorus concentrations is indicated in cats with CKD and is also often part of a wellness exam. While blood phosphorus is hormonally regulated, it can be influenced not only by the form and amount of dietary phosphorus, but also by a natural daily cycle known as a circadian rhythm. The blood phosphorus concentrations vary up to 30% between the highest and lowest points in humans. Further investigation is warranted in cats. The purpose of this study is to determine and characterize the circadian rhythm of phosphorus concentrations in healthy cats fed a consistent, complete, and balanced diet.