Sixty multiparous, lactating Holstein cows [57more or less 23.1. d in milk at the start of the experiment] were used in a completely randomized design to examine effects of adding incremental levels of dietary essential oil compounds [EO; 0, 200, 400 and 600. mg/d] on milk production and composition. Cows were allowed to graze on winter oats for 8. h/d with a daily herbage allowance of 15. kg dry matter [DM]/cow, and then received supplemental corn silage and sunflower meal in confinement for the remainder of the day. The EO were fed individually at milking times [0600 and 1600. h], mixed with 0.86. kg DM of dry rolled corn grain. In addition, 4 ruminally cannulated lactating Holstein cows in mid lactation were used in a 4x4 Latin Square design with 14. d periods to study effects of EO on ruminal fermentation characteristics and ruminal in sacco DM, crude protein [CP], and neutral detergent fiber [aNDF] degradability. Milk production, which ranged from 18.8 to 20.2. kg/d, and milk composition were not affected by EO. In general, ruminal fermentation characteristics were not affected by EO addition at any level, except for a 13 percent increase in butyrate concentrations with all EO levels compared to the control. Ruminal ammonia N concentration was high in all treatments [51.5 more or less 5.75. mg/100. ml] and tended [P=0.09] to increase when 200 and 400. mg/d of EO were added. In addition, 200. mg/d of EO marginally decreased the potentially ruminally degradable fraction of the CP of the complete ration. Results using dairy cows in mid lactation that grazed 8. h/d on lush pasture showed limited effects of this EO complex on ruminal fermentation, milk production and milk composition.