The metabolism of Mg was studied in young dairy calves fed two levels of added Al (0 and .20% Al) and two levels of added P (0 and .22% P) for 7 wk. The four treatments were 1) normal P-low Al, 2) low P-low Al, 3) normal P-high Al and 4) low P-high Al. The basal diet (low P-low Al) contained, by analysis, .132% P, .021% Al and .17% Mg. Added Al did not affect (P greater than .10) serum Mg. An Al x P interaction on bone Mg was detected (P less than .01). Magnesium was reduced in tibia shaft (.34 vs .44%) and in tibia joint (.43 vs .53%) in calves fed high Al in the presence of normal dietary P, but Mg was not reduced in the calves fed low-P diets. Apparent absorption of Mg was reduced by approximately five-fold (.18 g/d vs -.84 g/d, P less than .01); urinary Mg excretion was reduced 31% (1.12 g/d vs .77 g/d, P less than .01); and Mg retention declined 41% (-95 g/d vs -1.61 g/d, P less than .01) in calves fed added A1. Compared with calves fed low-P diets, calves fed normal levels of P had a higher Mg concentration in tibia shaft (P less than .01) and tibia joint (P less than .05). The data indicate that supplemental Al may adversely affect Mg metabolism in calves.