Prolyl endopeptidase (PEP, EC 3.4.21.26) has been proposed to play a role in degradation of proline-containing neuropeptides involved in the processes of learning and memory, e.g., vasopressin, substance P, and thyrotropin-releasing hormone (TRH). In the course of our search for bioactive constituents in medicinal plants, we studied the PEP inhibitory constituents of the roots of Lindera strychnifolia F. VILL and isolated two known tannins, epicatechin (1) and aesculitannin B (2), and four known sesquiterpenes, linderene (3), linderene acetate (4), linderalactone (5) and isolinderalactone (6) as inhibitors. On the inhibitory activities of six compounds against PEP from Flavobacterium meningosepticum and that from rat brain supernatant, compounds 1, 2 and 4 inhibited the enzyme from Flavobacterium more strongly than that from rat brain supernatant. However, compounds 3, 5 and 6 inhibited the enzymes from both origins to the same extent and furthermore, compound 6 was the strongest natural inhibitor against PEP from rat brain supernatant. The kinetic study of these inhibitors indicated that compounds 1, 2 are noncompetitive inhibitors and compounds 3—6 are competitive inhibitors. This is the first example of non-phenolic constituents showing significant competitive inhibitory activity being isolated from natural medicines.