Two diastereomeric furan-2-carbonylamino-3-oxohexahydroindolizino[8,7-b]indole carboxylates, highly constrained analogues of endogenous pyroglutamyl tripeptide inhibitors of snake venom endopeptidases, have been prepared as potential inhibitors of adamalysin II and matrix metalloproteinases. They proved to be inactive against adamalysin II and weak inhibitors of gelatinase A, gelatinase B, stromelysin 1 and human neutrophil collagenase. Evaluation of the mode of binding of the (2R,5S,11bR) isomer in the active site of adamalysin II suggests that the decrease of potency may be due to the reorientation of the acylamino chain in three of the heterocyclic nucleus, to a short contact at the entrance of the S'(1) hydrophobic cleft and to the loss of flexibility of the tetracyclic nucleus in the P'(1), P'(2) region of the inhibitor, which prevents optimal arrangement in the S'(1) specificity subsite.