The effects of high protein feeding on food and water intake, and the retention and urinary excretion of macrominerals (magnesium (Mg), phosphorus (P), and calcium (Ca)) were examined in growing cats. Seven female cats aged 4 months were fed diets containing 55% crude protein (n = 4) or 29% crude protein (n = 3) for 12 months on an ad libitum basis. Mineral balances were determined at 0.5, 2, 6, 10, and 12 months of feeding. The higher protein intake stimulated daily water intake and urine excretion throughout the study, although daily food intake was not affected by dietary protein levels. The urinary Mg concentration was decreased by the high protein intake, resulting from both increased urine volume and reduced excretion of urinary Mg. In contrast, the concentration and daily excretion of urinary P were increased by the high protein intake. The protein-induced increase in urinary P would not necessarily imply the increased excretion of PO4(3-), the anion responsible for struvite crystallization, because the dissociation of phosphate depends on urinary pH. Urinary Ca excretion was not affected by the dietary protein levels, but the high protein intake caused less retention of P and Ca as a result of enhanced urinary P excretion and lowered Ca absorption. The possibility of high protein feeding for the prevention of struvite crystallization in growing cats is discussed.