Activation of the contact phase of coagulation has been implicated in the pathogenesis of septic shock. We wanted to determine if inhibition of plasma kallikrein can prevent arterial hypotension and liberation of kinins from kininogen, induced by an infusion of bacterial lipopolysaccharide (LPS) in anesthetized, ventilated 20-kg pigs. The LPS was given IV in a dose of 5 [mu] g/kg/h for 8 hours. The plasma kallikrein inhibitor aprotinin, 537 [mu] mol, was given IV during 8 hours, resulting in plasma levels above 10 [mu] mol/L. Ten animals (SA) received LPS and aprotinin and ten randomized controls (SC) received LPS and saline. Kinin-containing kininogen was determined on the basis of the amount of kinin releasable in plasma samples by incubation with trypsin. Kininogen decreased to 58%+/-4% of the baseline value without any difference between groups. This may indicate participation of other processes than degradation by plasma kallikrein in the decrease of kininogen. Arterial blood pressure was higher at 7 hours in the SA animals than in the SC group (101%+/-11% vs. 68%+/-8%; mean+/-SEM; p= 0.026). Fibrin monomer and C3adesArg plasma levels were attenuated by aprotinin treatment. These findings underscore the important role of the contact system in LPS shock.