Abstract: Intravenuous injection of E. coli endotoxin 30mg/kg to rats resulted in a systemic hypotension and rapid decrease in mesenteric arteriolar blood flow velocity (MABFV). The mesenteric arterioles constricted within 3 min followed by a short period of vasodilation after endotoxin administration. Pretreatrnent with the platelet-activating factor receptor antagonist SRI 63-441 attenuated the endotoxin-induced systemic hypotension and the decrease in MABFV, and abolished the mesenteric vasodilation effect of endotoxin infusion. The results suggest that PAF may be a mediator of endotoxic shock.