Purpose The tryptophan-containing dipeptides isoleucine-tryptophan (IW) and tryptophan-leucine (WL) are angiotensin-converting enzyme (ACE)-inhibitors in vitro. These peptides are released by enzymatic hydrolysis of bovine whey protein. To exhibit ACE inhibition in vivo, peptides need to be absorbed into the circulatory system. This study aimed to determine the in vivo ACE-inhibitory potency of a whey protein hydrolysate (MPH), containing IW and WL, and to quantify plasma concentrations of these peptides after oral administration of MPH in healthy volunteers. Additionally, changes in blood pressure were investigated. Results After intake of 5 and 50 g MPH, plasma ACE activity was reduced to 86.4 +/- 5.9 and 75.1 +/- 6.9% of baseline activity, respectively. Although a clear ACE inhibition was measured, no effect on blood pressure was seen. Basal plasma concentrations of the tryptophan-containing dipeptides were 2.8 +/- 0.7 nM for IW and 10.1 +/- 1.8 nM for WL. After intake of 5-50 g MPH, peptide concentrations were dose dependently elevated to values between 12.5 +/- 8.4 and 99.1 +/- 58.7 nM for IW and 15.0 +/- 4.3-34.9 +/- 19.4 nM for WL. Administration of intact whey protein showed a minor ACE inhibition, probably caused by release of inhibitory peptides during gastrointestinal digestion. The increase of WL in plasma after intake of intact protein was similar to that determined after intake of MPH. In contrast, resulting IW concentrations were much lower after intake of intact whey protein when compared to MPH administration. Conclusion After intake of MPH, plasma ACE activity decreased in parallel to the increase of IW and WL plasma concentrations. However, the resulting peptide concentrations cannot fully explain the reduction of ACE activity in plasma with a direct enzyme inhibition. Therefore, this study points to a gap in the understanding of the inhibitory action of these peptides in vivo. Thus, to further develop innovative food additives with ACE activity diminishing capabilities, it appears mandatory to better characterize the mode of action of these peptides.