Several milk proteins are precursors of peptides, released during fermentation, which possess various biochemical and physiological properties. Among them, some peptides have been reported to inhibit angiotensin-converting enzyme (ACE) in vitro (an enzyme implicated in blood pressure regulation) and to lower blood pressure (BP). in hypertensive rats and in a small sample of patients with hypertension. One possible mechanism for the BP lowering effect is inhibition of ACE. Prior investigating BP lowering effect of these peptides in a large cohort of hypertensive patients, it is necessary to assess in depth their potency to inhibit ACE in vivo in humans and to compare it to that induced by a well-known ACE inhibitor.