Monensin was fed at levels of 0 and 33 ppm in a series of digestion and metabolism trials to determine its effect on utilization of high grain diets and on ruminal parameters in yearling steers. Monensin had no effect (P greater than .05) on apparent digestibility of dry matter, gross energy or starch when fed with a 90% corn diet (10.5% crude protein, dry matter basis). Monensin in this corn-based diet tended to increase crude protein digestibility (63.4 vs 61.3%) and decrease ruminal ammonia concentration (2.5 vs 6.5 mg/100 ml) measured 3 hr postprandially (P greater than .05). In metabolism trials with a 76% sorghum grain diet (11.7% crude protein, dry matter basis), monensin improved apparent digestibility of crude protein (P less than .05) but not dry matter or gross energy (P greater than .05). Retention of nitrogen, expressed as a percentage of nitrogen intake, tended to improve (24 vs 20%) in response to monensin addition to the sorghum grain-based diet (P greater than .05); a similar trend was observed (P greater than .05) for nitrogen retention expressed as a percentage of nitrogen absorbed (41 vs 36%). Total ruminal volatile fatty acid (VFA) concentrations measured 3 hr postprandially were not altered by monensin fed with either diet. The proportion of acetic acid d-creased (P less than .05) and that of propionic acid increased (P less than .05) in response to monensin fed with the corn-based diet. Monensin did not appreciably alter proporations of VFA when fed with the sorghum grain-based diet. Results suggest that a possible improvement in N utilization may account for some of the benefits of feeding monensin with high grain diets.