Ten juvenile pigs receiving a continuous infusion of 0.01 mg/kg of endotoxin over 3 hr and seven animals infused with sterile saline (serving as controls) were studied for 5 hr. Endotoxin concentrations in plasma as determined with a chromogenic Limulus amoebocyte lysate (LAL) test reached a steady state of about 1,000 ng/liter after 1 hr and declined rapidly as the infusion was discontinued. Preinfusion values were reached at the end of the observation period. Endotoxin concentrations found during the infusion period were comparable with those seen in humans with septicemia. The endotoxin infusion was followed by hemoconcentration, leukocytopenia, and thrombocytopenia. Using chromogenic peptide substrate assays, activation of the plasma kallikrein-kinin, fibrinolytic, and coagulation systems was detected. Although the endotoxin concentrations reached preinfusion values within the last 2 hr of the observation period, changes found in circulating cells and components of the plasma cascade systems did not normalize, and the hemodynamic situation did not change.