A series of potent and selective prolylendopeptidase (PEP) inhibitors of the alpha-keto heterocyclic type has been obtained by replacing the classical central proline of 1-[1-(4-phenylbutanoyl)-L-prolyl]pyrrolidine (SUAM 1221,3) by non-natural amino acids PHI, ABO, and ABH. These 4-phenylbutanoyl side-chain-containing inhibitors exhibited potent in vitro inhibitory potencies with IC50 around 30 nM (compounds 24 and 25). Modulation of the side chain by replacement of the terminal phenyl ring by the dicyclopropyl moiety afforded derivatives 30 and 32 with improved potencies (IC50 between 10 and 20 nM). Furthermore, replacing the linear 4-phenylbutanoyl side chain by the (2-phenylcyclopropyl)carbonyl entity provided potent inhibitors with IC50 culminating at 0.9 nM on a rat cortex enzymatic preparation (compound 70). The configuration of the cyclopropyl ring had to be R,R in order to obtain not only a strong PEP inhibition in vitro but also a good activity in vivo, exemplified by inhibitor 68, which gave ID50 ip and po of 0.3 and 1 mg/kg, respectively. Finally, demonstration of the cognition-enhancing properties of compound 54 was given in the passive avoidance test using scopolamine-induced amnesia in the rat, where it dose dependently inhibited the scopolamine-induced decrease in avoidance response.