Phosphorus and calcium deficiency in horses represents an important factor responsible for the low equine production in Brazil. The basic mechanisms of P and Ca metabolism differ substantially among species. Regulation of P and Ca metabolism is less well understood in horses than in others species. With the use of the isotopic dilution technique is possible to evaluate the metabolism for this mineral. The aim of the present experiment was to study the effect of different Ca levels in the diet on P and Ca metabolism in horses.