Duodenal flows of hydrogenation intermediates in response to changes in dietary forage:concentrate ratio (F:C) and linseed oil were evaluated using 4 lactating Holstein cows fed a low (65:35 forage to concentrate) or high (35:65) concentrate diet without (LC, HC) added oil or with linseed oil (LCO, HCO) at 3% of DM. A 4 x 4 Latin square design was implemented for 5 wk. Lower hydrogenation of 18:2n-6 and 18:3n-3 was observed with HC, but it increased with LCO or HCO. Duodenal flow of total conjugated linoleic acids (CLA) increased by 1.40 (LCO) to 3.01 (HCO) g/d with linseed oil. This response was associated with greater flows of cis9,trans11- (+0.21 to +0.55 g/d), trans11,cis13- (+0.33 to +0.36), trans11,trans13- (+1.01 to +1.15 g/d), and trans,trans-CLA (+0.12 to +0.72 g/d). Trans10,cis12-CLA flow averaged 0.08 g/d and was not affected by F:C or oil. trans11,cis15-18:2 flow increased by 8.5 (LCO) to 62 (HCO) g/d in response to linseed oil. Total trans-18:1 flow was 37 g/d in cows fed LC and increased to 81 g/d with HC. Feeding oil increased total trans-18:1 to the greatest extent with HCO. Flow of trans10-18:1 was lower with LC than with HC (1.46 vs. 20 g/d). Linseed oil increased trans11-18:1 flow by 40 (LCO) to 113 g/d (HCO). Feeding LCO and HCO also increased flows of trans6+7+8-, trans13+14-, trans15-, and trans16-18:1. Apparent intestinal digestibility of trans-18:1 isomers was largely unaffected by concentrate level and ranged between 67 and 95%. Linseed oil increased digestibility of nearly all isomers by 3 to 16 percentage units. Digestibility of cis9,trans11-CLA was greater in cows fed HC (55%) compared with cows fed LC (32%) and was not affected by linseed oil. Data suggest that high concentrate diets enhanced ruminal outflow of trans10-18:1. We provide initial in vivo evidence that supplemental 18:3n-3 is hydrogenated to trans11,cis15-18:2, trans11-18:1, trans13+14-18:1, trans15-18:1, trans6+7+8-18:1, and trans16-18:1 primarily.