Rumen simulating techniques (Rusitec) were used to determine the impact of diets containing milled oilseeds on the fermentation parameters and amount of fatty acids (FA) in the effluent. High-forage diets containing no oilseeds (control diet (CD)) or 10% oilseed meal from rapeseed (RS), sunflower seed (SS), or flaxseed (FS) were used on a dry matter (DM) basis. No differences in DM digestibility were observed between the diets. Inclusion of SS and FS significantly reduced the pH values of the ruminal fluid, and a significant decline in the ammonia nitrogen (NH3-N) (mg/d) production in effluent was observed in the vessels with SS. Generally, oilseeds in these diets significantly reduced the amount of total fermentation gases (L/d); however, only a tendency toward methane (CH4, %) decrease was detected. The addition of oilseeds also significantly diminished the amount of total volatile fatty acids (VFA) produced (mmol/d). Significant reductions in the amounts of saturated FA in the vessels with RS and FS were observed compared with the CD and a significantly higher amount of monounsaturated fatty acids (MUFA) was noted in the vessels with RS. An increased amount of polyunsaturated fatty acids (PUFA), compared with the CD, was statistically significant only in the vessels with FS.