Aminopeptidase P (EC 3.4.11.9) was purified from rat brain cytosol. A subunit Mr of 71000 was determined for the reduced, denaturated protein whereas an Mr of 143 000 was determined for the native enzyme. The purified aminopeptidase P selectively liberated all unblocked, preferentially basic or hydrophobic ultimate amino acids from di‐, tri‐ and oligopeptides with N‐terminal Xaa‐Pro‐ sequences. Corresponding peptides with penultimate Ala instead of Pro were cleaved with much lower rates; oligopeptides with residues other than Pro or Ala in the penultimate position appeared not to be substrates for the enzyme. Several bioactive peptides with Xaa‐Pro sequences, especially bradykinin, substance P, corticotropin‐like intermediate lobe peptide, casomorphin and [Tyr]melanostatin were shortened by the N‐terminal amino acid by aminopeptidase P action. Rat brain aminopeptidase P was optimally active at pH 7.6–8.0 in the presence of Mn2 +. Chelating agents and SH‐reacting reagents inhibited the enzyme, but common inhibitors of aminopeptidases, like amastatin or bestatin, of prolidase or of dipeptidyl peptidases II and IV, like N‐benzoyloxycarbonyl‐proline or ɛ‐benzyl‐oxycarbonyl‐lysyl‐proline, as well as antibiotics like β‐lactam ones, bacitracin or puromycin, had little or no effect.