In the modification of the fibrinogen fragment related sequences ARPAK, QRPAK GRPAK and KRPAK, the corresponding cyclo-ARPAK, cyclo-QRPAK, cyclo-GRPAK, and cyclo-KRPAK were prepared in the diluted solution. The bioassay in vivo indicated that the thrombolytic potencies of cyclo-ARPAK, cyclo-GRPAK, cyclo-QRPAK, and cyclo-KRPAK were significantly higher than that of ARPAK, QRPAK, GRPAK, and KRPAK. In water, the cyclopeptides were incubated with pepsin or trypsin at 37 degrees C for 64 h. There was no degradation product observed, on the other hand, with the same condition, the peptides were completely hydrolyzed in 8 h. The relationships among the rigidity or the conformation and the thrombolytic activity in vivo and the stability to enzyme-induced hydrolysis in vitro of the cyclopeptides were discussed.