Aminopeptidase P (APP) is metallopeptidase that specifically hydrolyses the N-terminal amino acid residue from peptides presenting a proline at P1'. The enzyme is ubiquitously expressed being found in several organisms including bacteria, yeast and vertebrates. Human APP exists as a membrane-bound (m-APP) and a cytosolic homologue (c-APP). Despite of its putative involvement in the processing of bioactive peptides, little is known of the physiological roles of both forms of APP. In human plasma, the enzyme is involved in the processing of bradykinin (BK). However, in the presence of ACE inhibitors APP is the main metabolic pathway of BK 1–8 and the only degrading enzyme in vivo of this peptide. We performed a comparative study of catalytic and physicochemical properties of human recombinant m-APP and c-APP using as substrates internally quenched fluorescent BK derivatives containing o-aminobenzoic acid (Abz) as fluorescent group and 2,4-dinitrophenyl (Dnp) or (2,4-dinitrofenil)-etilenodiamino (EDDnp) as quencher. The peptide NH₂-K(Dnp)-PPGK-Abz-NH₂, hydrolyzed by both forms with the highest catalytic efficiency, was used in the studies of the influence of pH, salt, thermal decomposition and stability of the enzymes. The effects of Cl^- and pH were very similar for both forms. The c-APP completely lost activity after lyoplylisation while m-APP did not change the catalytic properties after this procedure. Further studies are now in process to better characterize the differences in substrate specificity and in thermal stability of the enzymes. Support: CNPq and  FAPESP.