Twenty-seven Holstein cows were fed total mixed rations (TMR) supplemented with protected palm fat (PPF), rapeseed cake (RC) or extruded soyabean (ESB) for 14 weeks. Feed intake and milk yield were recorded. Samples of milk were taken weekly and analysed. Jugular blood was taken three times two hours after feeding. Cows of the PPF group consumed significantly less feed and produced more milk than cows of the RC and ESB groups (32.9, 31.7 and 30.7 kg fat-corrected milk/d, respectively). Milk fat from cows fed diets supplemented with RC and ESB contained more stearic, vaccenic and conjugated linoleic acids, and less palmitic acid than milk fat of cows fed the PPF diet (P<0.05). The content of polyunsaturated fatty acids (PUFA) in milk fat of cows fed ESB was higher, and the atherogenicity of its fatty acids, lower than in milk fat of other cows. This was, however, accompanied by lower oxidative stability of milk fat. Supplementation of TMR with RC increased the spreadability index of manufactured butter (P<0.025). Both milk and blood urea concentrations were significantly lower for cows fed the ESB diet. This suggests that crude protein in this diet was less degradable than in other diets. Body weights of cows on diets RC and ESB increased by 11.3 and 15.6 kg, respectively, whereas the body weight of cows on the PPF diet decreased by 1.7 kg, on average. It can be concluded that feeding oilseed-based supplements to dairy cows decreased milk yield and milk production efficiency somewhat, but improved the nutritional quality of milk fat.