Endothelial cells (EC) synthesize platelet-activating factor (PAF) when activated by agents such as ATP or thrombin, and PAF production occurs as a consequence of endothelial phospholipase A activity. Because interactions between Rickettsia prowazekii and a variety of host cells result in the expression of phospholipase A activity, we assessed the relative abilities of uninfected and rickettsia-infected EC to synthesize PAF. Endothelial cells were infected with rickettsiae and examined at 24-h intervals for rickettsial multiplication, EC viability, and PAF synthesis. By 24 h postinfection, 80% of the EC were infected with an average of 10.6 rickettsiae per cell; by 72 h, the rickettsiae were too numerous to count and the numbers of viable EC began to decrease. Both rickettsia-infected and sham-treated EC synthesized PAF when stimulated with either thrombin or ATP, but rickettsia-infected EC synthesized about three times as much PAF in response to cell activation as did their uninfected counterparts. Additionally, unlike their uninfected counterparts, rickettsia-infected EC synthesized significant amounts of PAF in the absence of cell activation; rickettsia-infected EC synthesized as much PAF in the absence of activation as did uninfected EC in response to ATP. In each case, essentially all of the newly synthesized PAF remained with the cell pellet. Finally, EC incubated with high numbers of rickettsiae (1,000 rickettsiae per EC) for 30 min synthesized more PAF when activated with ATP than did their sham-treated activated counterparts.