Human renin occurs in a latent form as prorenin in blood and amniotic fluid. We found that the metalloprotease thermolysin is a more potent activator of amniotic and plasma prorenin than trypsin, provided the thermolysin alpha 2-macroglobulin plasma inhibitor was inactivated. Thermolysin fully activated amniotic prorenin at a 23-fold lower molar concentration than trypsin, and renin activated by thermolysin was more stable than when activated by trypsin. Thermolysin also activated plasma prorenin at a 16-fold lower concentration than trypsin in the presence of methylamine (100 mM). Thermolysin activated prorenin directly, because added inhibitors of other endogenous proteases did not block the activation. The maximum activation values obtained after incubation with trypsin or thermolysin in plasma samples from 17 normotensive and 58 hypertensive subjects were similar. The mean renin concentration did not differ significantly in normotensives and hypertensives, but after activation, total renin was significantly higher in hypertensive subjects (89.8 vs. 53.4 ng angiotensin I/h . ml). The Km of the substrate, angiotensinogen, was about the same with both active renin and activated prorenin (281-290 nM). The mol wt of prorenin was 60,000 after gel filtration; activation by thermolysin reduced it to 51,000. Thus, thermolysin, which has a different peptide bond specificity than trypsin, is another model for a prorenin activator.