The objective of this study was to determine rumen degradability and total digestibility of flavonolignans from a milk thistle fruit expeller in dairy cows considering milk production and changes in plasma flavonolignans. The experiment was carried out on three lactating Holstein cows and was divided into three periods as follows: preliminary period (Pr, 3 days) was used for the diet stabilization followed by the adaptation period (A, 6 days) in which the treatment was applied and by the balance period (B, 4 days). Cows were fed individually twice a day (6:40 and 16:40 h) ad libitum the diet based on maize silage, lucerne hay and supplemental mixture. In the periods A and B the diet was supplemented with 150 g/day of milk thistle fruit expeller applied in two equal portions at each feeding. Average daily intake of dry matter and basic nutrients was similar in all periods (P > 0.05). Milk yield and composition were not affected by the treatment (P > 0.05). The milk thistle fruit expeller used in this experiment contained 4.10 ± 0.10 mass percentage of the silymarin complex. Digestibility of silybin A and silybin B was 40.0 and 45.5%, respectively. Digestibility of other components of the silymarin complex was 100%. The highest value of the effective degradation was found for taxifolin (59.11%), while the effective degradation of the other flavonolignans ranged from 23.28 to 35.19%. Animals receiving the milk thistle fruit expeller had a higher content of plasma conjugated silybin (P < 0.001) than those without its supplementation.