Technical Abstract: A four-unit continuous culture fermentor system was used to evaluate the effects of oilseed supplementation of an herbage-based diet on ruminal fermentation. Treatments were randomly assigned to fermentors in a 4 x 4 Latin square design with 7 d for diet adaptation and 3 d for data and sample collection. Dietary treatments were: herbage-only diet (HERB), or the following oilseeds supplemented to an herbage- based diet at 10% of total DM fed: flax (FLAX), canola (CAN) or sunflower (SUN). Apparent DM, OM and NDF digestibilities were not affected by diet, averaging 62.2, 67.9 and 78.1%, respectively. True DM and OM digestibilities were not affected by diet, averaging 77.9 and 82.4%, respectively. Fermentor pH and total VFA were not affected by diet. Branch-chained VFA tended to be lower for HERB. Ammonia concentrations were least for HERB. The CP digestibility was not affected by diet. Flows of NH3-N were least for HERB. Microbial N flows were least for SUN and HERB, and greatest for CAN. Flows of total N, non-NH3-N, and dietary N were not affected by diet. Efficiency of microbial N synthesis was not affected by diet. Supplementation with oilseeds at 10% of total DM fed did not impact nutrient digestibility or ruminal fermentation compared with an all-herbage diet. These results suggest that these oilseeds may be considered as an alternative energy supplement for grazing dairy cows, particularly during times of high prices or low availability of corn. However, in vivo studies are needed to quantify the effects of oilseed supplementation on milk production and milk composition (particularly beneficial fatty acids) of an herbage-based diet.