In this study, rapeseed protein isolate (RPI) was digested with various proteases to produce rapeseed protein hydrolysates (RPHs), which were then separated into different peptide fractions (<1, 1'3, 3'5, and 5'10 kDa) by membrane ultrafiltration. Membrane fractionation showed that peptides with sizes <3 kDa had significantly (p < 0.05) reduced surface hydrophobicity when compared to the RPHs and peptide fractions with sizes >3 kDa. In contrast, the <3 kDa peptides showed significantly (p < 0.05) higher oxygen radical scavenging ability when compared to the >3 kDa peptides and RPHs. In vitro inhibition of angiotensin I-converting enzyme (ACE) was significantly (p < 0.05) higher for the Thermolysin, Proteinase K and Alcalase RPHs when compared to the pepsin + pancreatin (PP) and Flavourzyme RPHs. The Alcalase RPH had significantly (p < 0.05) higher renin inhibition among the RPHs, while with the exception of Thermolysin, the 5'10 kDa peptide fraction had the least renin-inhibitory ability when compared to the <5 kDa peptide fractions. Oral administration (100 mg/kg body weight) of the RPHs and RPI to spontaneously hypertensive rats (SHR) showed the Alcalase RPH to be the most effective in blood pressure (BP) reduction (~24 mm Hg) while Proteinase K RPH was the least effective (~5 mm Hg) after 8 h. However, the PP RPH had the most prolonged effect with BP reduction of ~20 mm Hg after 24 h of oral administration. We conclude that the strong BP-lowering ability of Alcalase and PP RPHs could be due to high resistance of the peptides to structural degradation coupled with high absorption rate within the gastrointestinal tract.