Thirty-two lactating dairy cows, divided into four groups of eight cows each, were fed from day 10 to 110 of lactation rations composed of maize silage, meadow hay and concentrate mixture (control group K), with an addition of ground rape seeds var. Jantar OO: unprotected (group RN), treated with formaldehyde (group RF) or heat-treated (group RO). The daily ration of ground seeds was 1.5 kg per cow. In addition, all of the cows in each group received extra concentrate mixture in amounts depending on their milk yield. Cows from group K had a lower FCM (approximately 21 kg per day (P≤0.05) than those in groups RF and RO (about 23 kg each). Milk yield in group RN (21.5 kg) did not significantly differ from the others. The difference in the percentage fat content in the milk was statistically significant (P≤0.05) only between group K (3.88) and RF (4.29). The milk from cows in groups RF and RO contained less protein than the others, but these differences were not statistically significant. The unsaturated fat content in the milk fat of cows receiving rape seeds was higher, especially in group RF, than in the control.