Forty-eight Angus and Hereford cows were used in a completely random design with a 2 X 2 factorial arrangement of treatments to evaluate the effects of monensin and two sources of Mg on performance, ruminal characteristics and mineral status of mature beef cows. Cows were fed wheat straw treated with NH3 during late gestation and straw treated with NH3 plus bromegrass haylage after calving along with a protein supplement that contained either MgO (7.5 g/kg) or MgSO4 (11 g/kg). One half of the cows received monensin (1 g/kg) in their protein supplement. During the precalving period, MgO without monensin increased (P less than .01) plasma Mg compared with Mg in plasma of cows receiving MgSO4 or MgO with monensin. Magnesium oxide also increased cow weight gains during late gestation (P less than .05) compared with weight gains of cows supplemented with MgSO4, but magnesium source had no effect on weight gain postcalving. Plasma glucose was increased by monensin (P less than .05) and by MgSO4 (P less than .01) during early lactation. Monensin increased the proportion of propionate before (P less than .05) and after (P less than .10) calving in the ruminal fluid of cows. Monensin also increased (P less than .05) cow weight gain precalving and tended to decrease (P less than .10) milk fat in lactating cows. Calf weight gains were not affected by treatment of dam. Magnesium oxide appeared to have higher Mg bioavailability than MgSO4 for cows precalving, but Mg bioavailability was not different postcalving. Monensin increased cow weight gains precalving and improved feed efficiency postcalving.