Green tea (Camellia sinensis L.) and Rooibos (Aspalathus linearis Dahlg.) inhibit angiotensin-converting enzyme (ACE) in vitro and in vivo. The ACE inhibitor enalaprilat has been described previously as a competitive inhibitor and sometimes as a non-competitive inhibitor. The aim of this study was to investigate the pharmacological mechanism of ACE inhibition of green tea and Rooibos by enzyme kinetics, and to compare this with enalaprilat. A Michaelis-Menten kinetics and Lineweaver-Burk graph showed mean values of V(max) = 3.73 µM and K(m) = 0.71 µM for green tea, of V(max) = 6.76 µM and K(m) = 0.78 µM for Rooibos, of V(max) = 12.54 µM and K(m) = 2.77 µM for enalaprilat, and of V(max) = 51.33 µM and K(m) = 9.22 µM for the PBS control. Incubating serum with green tea or Rooibos saturated with zinc chloride did not change the inhibitory effect. Enalaprilat preincubated with zinc chloride showed a decrease in the inhibitory effect. In conclusion, green tea, Rooibos and enalaprilat seem to inhibit ACE activity using a mixed inhibitor mechanism.