Milk and dairy products considerably contribute to the nutritional value of human diets. In order to benefit human nutrition bovine milk fatty acid composition and phenol content are effectively manipulated by the cow’s diet. However, response times taken for these alterations to occur have not been quantified. In the present study, fatty acid composition and phenol content of the milk were evaluated after three days of feeding six cows six different diets, supplemented with six different tanniferous plants (hazel, silver birch, blackcurrant, grape vine, wood avens and rosebay willow with total tannin concentrations of 26, 36, 42, 52, 55 and 79 g/kg dry matter, respectively). Lucerne was applied as the low-phenol control diet. Substantial changes in total phenols and fatty acids were found in milk samples after just three days. Proportions of cis-9 trans-11 C18:2 and trans-11 C18:1 declined by 29 and 68%, respectively, in comparison to milk from cows fed lucerne, indicating a definitive ruminal biohydrogenation response. However, there were no significant effects between test plants and lucerne when comparing C18:3 n-3 and C18:2 n-6 proportions in milk fat. So, it was demonstrated that phenols and certain individual fatty acids in bovine milk can be rapidly modified by adding specific tanniferous plants to the diet.