Human nutritionists have identified that dietary myristic and palmitic fatty acids are major risk factors in cardiovascular disease. Bovine milk fat is particularly rich in these fatty acids as they can be produced de novo in the mammary gland from by-products of fibre fermentation in the rumen. However, increased dietary levels of C18 fatty acids has been shown to reduce the concentration of myristic and palmitic acids in milk fat. Supplying dietary oil from whole oilseeds treated with xylose, may reduce rumen biohydrogenation of polyunsaturated fatty acids and further decrease the saturated fatty acid content of milk. In this study, the effect of feeding whole ground soya beans, whole ground soya beans treated with xylose and whole ground rape seed on dairy cow performance and milk composition was investigated. All three oilseeds reduced dry matter intakes and milk protein concentration. Both the xylose treated whole ground soya beans and the whole ground rape seed increased milk yield, but the whole rape reduced milk fat content. Levels of linoleic acid in milk fat increased when whole soya was fed, whereas oleic acid concentration was highest when whole rape was included in the diet. Concentrations of myristic and palmitic acids in milk fat were lower when any whole ground oilseed was fed, but the reduction was greatest with whole ground rape which reduced the myristic plus palmitic acid content of milk fat to 25 g/100 g fatty acid, compared with 47 g/100 g milk fatty acid in the milk from cows fed a standard low oil diet. These results suggest that feeding specific sources of dietary oil can dramatically reduce levels of myristic and palmitic acids in milk fat, and that different oilseeds have different effects on milk fat concentration.