Airborne polycyclic aromatic hydrocarbons (PAHs) are deposited on agricultural grasses. In turn, PAHs enter the food chain through animals eating grasses. However, the risk of food contamination, e.g. of milk, is unknown because mechanisms ruling the fate of PAHs during digestion by cows are not understood, especially in the rumen. Here, we studied the disappearance rate of phenanthrene, pyrene and benzo[a]pyrene, n-alkanes, and dry matter from contaminated grass samples in the rumen, the first compartment of the ruminant digestive tract. Three cows fitted with a rumen cannula were used for this study and the grass samples were incubated in sacco for 1, 3, 6, 12, 24 and 48 h. The results show that the disappearance rate of PAHs was fast and higher than that of n-alkanes. On average, 83% of PAHs disappeared after 1 h of incubation. Phenanthrene showed a disappearance rate higher than 90%. The findings suggest a fast desorption of PAHs from the surface of the grass leaves. The digestive hydrolysis of the grass is not required to release PAHs, contrary to other fractions. As a consequence, the bioavailability of PAHs is not limited by the grass.