Changes in endotoxin levels and cardiovascular performance during antibiotic therapy in septicemia were investigated in a porcine model. One group of animals (n = 9) received gentamicin 2 mg/kg intravenously infusion two hours after induction of sepsis with live E. coli bacteria. Another group (n = 7) served as non-treated septic controls. Plasma-levels of endotoxin increased significantly after antibiotic administration from 0.26 +/- 0.02 ng/ml before treatment (0 hrs), to 1.1 +/- 0.3 ng/ml after two hours (p less than 0.01) and 2.1 +/- 0.98 ng/ml four hours after treatment (p less than 0.01). In the control group no significant increase occurred in the observation period. No difference could be demonstrated between the groups with regard to the number of live bacteria in blood, either before or after treatment. When the data from all the animals were taken together for the first two hours following antibiotic administration a significant negative correlation (p less than 0.05) was demonstrated between changes in endotoxin levels and cardiac output. This correlation was significant for animals in which the levels of endotoxin increased above 0.5 ng/ml (p less than 0.05). The present study indicates that endotoxin is liberated after antibiotic administration during bacteremia, and that this increase correlates with cardiac performance.