This study reports the antihypertensive effect of orally administered doses of either Calpis sour milk or peptides (Val-Pro-Pro and Ile-Pro-Pro), which are inhibitors to angiotensin I-converting enzyme, isolated from the sour milk using strain SHR spontaneously hypertensive rats. Single oral administration of the sour milk (5 ml/kg of BW), corresponding inhibitory units of the peptides Val-Pro-Pro (.6 mg/kg of BW), or Ile-Pro-Pro (.3 mg/kg of BW) significantly decreased the systolic blood pressure from 6 to 8 h after administration. Blood pressure returned to the initial level at 24 h after administration. Antihypertensive activity of these two tripeptides was dose-dependent up to 5 mg/kg of BW. Conversely, the sour milk (25 ml/kg of BW) and mixed tripeptides (10 mg each of Val-Pro-Pro and Ile-Pro-Pro/kg of BW) did not change the systolic blood pressure of the normotensive strain WKY Wistar-Kyoto rats.