A 2 x 2 factorially designed experiment was conducted to study the effects of feeding two levels of calcium and phosphorus alone and in combination, on magnesium utilization in ruminants. Twelve wether lambs were used in two metabolism trials. The two levels of calcium were 0.4 and 1.4% and the two levels of phosphorus were 0.3 and 1.3%. Apparent magnesium absorption, expressed as percent of intake was lower for the high-calcium fed lambs (P< .05) and for the high-phosphorus fed lambs (P< .01). Urinary magnesium excretion was lower (P< .01) for the high-phosphorus fed lambs. Urinary calcium excretion was lower (P< .01) for the high-phosphorus fed lambs. Calcium retention was higher (P< .01) for the high-calcium fed lambs. Apparent phosphorus absorption, phosphorus retention and urinary phosphorus excretion were higher (P< .01) for the lambs fed the high-phosphorus level. Serum magnesium was lower (P< .01) and serum inorganic phosphorus was higher (P< .01) for the high-phosphorus fed lambs. Serum calcium was higher (P< .05) for the high-calcium fed lambs and lower (P< .01) for the lambs receiving the high-phosphorus rations. Apparent digestibility of ether extract, and dry matter content of the feces were higher (P< .05) for the lambs fed the high-calcium level and lower for the high-phosphorus fed lambs (P< .01). The significant (P< .01) interaction between calcium and phosphorus levels on dry matter of feces indicates that high dietary calcium was partially effective in reversing the detrimental effects of high-phosphorus level. / Master of Science