Effects of the administration of monensin via concentrates to dairy cows were studied in two trials. In one trial, 64 Holstein cows were assigned to four groups that received 0, 150, 300, or 450 mg/d of monensin from 5 to 24 wk postpartum. Milk production tended to increase (4.0, 3.3, and 5.4%, respectively) for the three groups of treated cows. Fat content was decreased by 0.09, 1.89, and 4.09 g/kg, respectively, for these same three groups. The effect on protein content was small and nonsignificant. Feed intake was reduced in treated cows, although not significantly, and feed efficiency was improved by monensin. In a confirmatory trial, 58 Holstein and 22 Jersey cows were allocated either to a control group or to a treatment group that received 300 mg/d of monensin from 5 to 36 wk postpartum during the first lactation and from 2 wk before calving to 36 wk postpartum during a subsequent lactation. During the first lactation, cows in the treatment group showed a 7% increase in milk production, a relative decrease (1.4 g/kg) in milk fat content, and equal protein content compared with cows in the control group. Body weight gain and body condition scores near the end of the treatment period were higher for cows in the treatment group. A decrease in blood ketone concentrations was found between 7 and 56 d of lactation. Treatment effects on milk production differed between breeds and within genetic lines. Jersey cows were less responsive than were Holstein cows, and Holstein cows with a high ratio of breeding values for protein and fat showed larger milk production responses. Results from the second lactation showed similar differences between the two groups as did those from the first lactation.