A hypotensive effect of intravenously injected des-Arg9-bradykinin was found in Brown/Norway strain young male rats which were pretreated with a small amount of endotoxin 24 h before the experiment, whereas the hypotensive effect of bradykinin was unaffected by the endotoxin. The potency of des-Arg9-bradykinin for the hypotensive effect was comparable to that of bradykinin. On the other hand, in endotoxin-pretreated aged rats, this effect of des-Arg9-bradykinin was not observed. Only during the intravenous infusion of des-Arg9[Leu8]bradykinin, a bradykinin B1 receptor antagonist, was the hypotensive effect of des-Arg9-bradykinin inhibited, whereas that of bradykinin was potentiated. After the end of infusion of des-Arg9[Leu8]bradykinin, the response to des-Arg9-bradykinin rapidly recovered. The results suggest the possibility that des-Arg9-bradykinin might play a role in inflammatory diseases.