The aim of the present study in cats was to investigate the potential effects of a calcium carbonate and chitosan supplement on blood parameters in aged cats with moderate chronic renal failure and on the mineral balance in adult healthy cats. For the trials, 10 neutered cats 2-4 years of age were fed for 21 days and six neutered cats (2 male and 4 female), 14 years of age, with elevated urea and phosphorus level in the plasma were fed for 35 days with a supplement. The apparent digestibility of phosphorus was (p < 0.05) reduced in the treatment period. Plasma urea inorganic phosphate decreased significantly (p < 0.05) in the old cats after 35 days of treatment. The treatment had a significant effect on the phosphorus, gross energy, dry matter, crude ash, crude fiber and crude protein digestibility in adult healthy cats. The practical implication could be an alternative treatment option for cats refusing to ingest veterinary renal diets.