The effect of adding evening primrose oil cake or ground full fat rape seed supplements to the diet on animal performance, hormone and metabolite levels in blood plasma, and milk fatty acid composition was compared in an experiment on 21 Black-and-White Lowland cows divided into 3 groups of 7. The animals were fed control basic diet (group C) or diets supplemented with evening primrose cake (group P) or full fat rape seeds (group R). Daily milk production in cows of group P was 0.9, and of group R 1.8 kg (FCM) higher than in cows of the control group. The milk fat contents were 3.99, 4.02 and 3.97%, and protein contents 2.98, 2.93 and 2.81% in groups C, R and P, respectively. The plasma cholesterol level was 20% lower in group C and 26% lower in group P than in group R. The content of C₁₈-acids, including polyunsaturated acids, in the milk fat of cows of group P and R was similar and higher than in controls.