A series of three experiments was conducted to quantify the impact of supplementing haycrop silage based diets for lactating dairy cows with ruminally protected methionine on intestinal disappearance of methionine and other amino acids and their net metabolism across the gastrointestinal tract and liver. The first trial demonstrated that the bioavailability of methionine from the source of ruminally protected methionine used was affected by residence time in the rumen and by the method used to assess its intestinal availability. The in situ mobile nylon bag technique underestimated the apparent disappearance of methionine in the small intestine compared with the in vivo method (44 vs 74%). The second experiment confirmed that, when determined in vivo, 66% of the methionine included in this ruminally protected source bypassed the rumen while 82% disappeared from the small intestine. On a net basis, nearly all of the methionine (99%) that disappeared from the small intestine was recovered in the mesenteric vein. However, only 66% of that methionine appeared in the portal vein indicating that the gastrointestinal tract is a net user of amino acids. Finally, in a third trial the addition of incremental amounts of ruminally protected methionine to the diet resulted in an increase in the net portal flux of methionine. However, liver removal of methionine increased from 25 to 36% of net portal flux leading to a non significant increase in the splanchnic release of methionine and to a significant increase in the arterial concentration of methionine. Interestingly, the addition of methionine had a positive effect on the net portal flux of the branched chain and non essential amino acids. The mechanism responsible for this phenomenon needs to be elucidated. Overall, the addition of ruminally protected methionine had little effect on milk protein production.