Eight crossbred steers (avg wt, 350 kg) with duodenal and ileal cannulas were used in two experimental periods in a single-reversal design to determine the effects of monensin on site and extent of apparent absorption of Mg and Ca. Steers were fed a cottonseed hull:concentrate diet (30:70) with or without 25 mg monensin/kg. The diet contained .20% Mg and .38% Ca. Each diet contained .25% Cr2O3 as a digesta marker. During each period, steers were fed the control diet for 10 d before allotment to treatments. Each period consisted of 17 d for diet adjustment and 6 d for collection. During the collection phase, fecal samples were obtained beginning on d 1 and continuing through d 6 at 12-h intervals progressing 2 h each day. Apparent Mg availability (grams apparently absorbed expressed as a percentage of grams of intake) increased (P less than .05) from 18.4 to 32.5% when monensin was fed. Feeding monensin increased (P less than .05) the apparent Mg availability and amount of Mg absorbed in the preintestinal region (47.7 vs 29.8% and 8.5 vs 5.1 g/d, respectively). Larger quantities (P less than .05) of Mg were secreted into the small intestine when monensin was fed. Monensin did not alter (P less than .05) apparent Ca availability or absorption in any segment of the digestive tract. These data indicate that monensin increased Mg and did not change Ca availability in the preintestinal region.