Results of this study show that an acute infusion of Bacteroides fragilis into a rabbit model is associated with a transitory and minimal lowering of mean arterial pressure. Peak left ventricular systolic pressure and its first derivative decreased significantly, but these abnormalities lasted for less than three minutes. By contrast, an infusion of Fusobacterium necrophorum is associated with a profound depression of mean arterial pressure, left ventricular systolic pressure and its first derivative, circulatory changes which are observed with a gram-negative aerobic infection. The lack of profound and permanent alterations in circulatory dynamics with the infusion of Bacteroides fragilis may be related to that organism's lack of classical endotoxin. While these studies were conducted in a nonprimate model, they should stimulate verification in a primate model. If validated, these results question the role of Bacteroides fragilis in producing septic shock.