The effect of long-term intake of two fermented milk products on the development of hypertension was compared in young spontaneously hypertensive rats (SHR). The products contained tripeptides isoleucine-proline-proline (IPP) and valine-proline-proline (VPP), which have been shown to possess angiotensin converting enzyme (ACE) inhibitory activity. Six-week-old SHR were divided into four groups to receive orally ad libitum water, skim milk or two fermented milk products (fermented milk A or fermented milk B; the latter is commercially available in Japan with trade name Calpis®) for 14 weeks. The calculated intake of IPP was 0·4 mg/d and 0·2 mg/d in the groups receiving fermented milk A and B, respectively, whereas the corresponding amounts for VPP were 0·6 mg/d and 0·3 mg/d. Systolic blood pressure (SBP) was monitored weekly by tail-cuff method. The development of hypertension was significantly attenuated in both groups receiving fermented milk products, whereas skim milk did not affect blood pressure. The effect was detectable after 6 weeks of treatment. At the end of the experiment, the lowest blood pressure level was found in the group receiving fermented milk A: the SBP was 21 mm Hg lower than in the group receiving water and 10 mm Hg lower than in the group receiving fermented milk B. This difference could be explained by larger intake of ACE inhibitory tripeptides in the group receiving fermented milk A as compared with fermented milk B.