In studying potential inhibitors of prorenin activation, we synthesized stereoisomers of a nonapeptide which spans the putative prorenin cleavage site. Peptide 67 has a D-Leu substitution on the amino side of the sessile bond and peptide 68 has a D-Arg substitution on the carboxy side. Both peptides equally inhibited human urinary kallikrein activity with an IC50 of 5 x 10(-4) mol/l. However, peptide 67 inhibited tryptic prorenin activation more effectively, with an IC50 of 5 x 10(-3) mol/l, than did peptide 68 with an IC50 of 10(-2) mol/l. Although the inhibitory properties of these peptides are weak, our data suggest that peptide analogue inhibitors of prorenin activation can be developed, and that prorenin activating enzyme(s) may be another potential target of renin-angiotensin inhibition.