: An angiotensin-converting enzyme inhibitory (ACEI) peptide with a median inhibitory concentration (IC₅₀) of 1.26 mg/mL was purified from whey proteins resulting from a fermentation using Lactobacillus plantarum QS670. The peptide was subsequently derived from an α_S1-casein, κ-casein, β-lactoglobulin, or serum albumin fraction. Analysis via liquid chromatography tandem mass spectrometry indicated that it had an amino acid sequence of Gly-Ala (GA). The GA dipeptide was also synthesized using an Fmoc solid-phase method. The GA dipeptide exhibited an IC₅₀ of 1.22 mg/mL and was shown to be stable across both temperature (20 to 60°C) and pH (2 to12). Digestive enzymes including pepsin, trypsin, and chymotrypsin had negligible effects on activity. The whey exerted hypotensive effects when fed to spontaneously hypertensive rats (SHR), which exhibited a blood pressure drop of 2.33 kPa. A 4-wk gavage treatment resulted in greater decreases of 7.46 kPa. Results of this study indicate that milk fermented using Lb. plantarum QS306 has potential to be used as a functional food to help prevent or reduce hypertension-associated diseases.