Inactive human renin is found in amniotic fluid, plasma, and kidney and may be a renin precursor ("prorenin"). The mechanism of activation of inactive renin in vivo is not known. The present study examined the hypothesis that cathepsin D, a lysosomal pepsin-like endopeptidase may be capable of eliciting activation. Cathepsin D was incubated with inactive renin in human amniotic fluid at pH 4.8 and 22 C for 0-5 h. Marked activation occurred and the reaction displayed first order kinetics with respect to the concentration of cathepsin D. The initial velocity of conversion of inactive renin to active renin by cathepsin D was 0.007%/min/microgram cathepsin D. Under identical conditions, the initial velocity of conversion by pepsin was 0.18%/min/microgram pepsin. The 25-fold higher potency of pepsin compared with cathepsin D is in accordance with the recognized relative substrate affinities and catalytic efficiencies of the two enzymes. Inactive renin in human amniotic fluid seems to be similar to that found in human kidney and since cathepsin D is present in juxtaglomerular cells, this activation process may have physiological importance.