Existing evidence suggests that oxidative insults and antioxidant defense mechanisms play a critical role in the host cell response during infection of endothelial cells by Rickettsia rickettsii, the causative agent of Rocky Mountain spotted fever. Heme oxygenase (HO), a rate-limiting enzyme in the pathway for heme catabolism, protects against oxidant damage in a variety of stress situations. Here, we report on the expression of the inducible and constitutive HO isozymes, HO-1 and HO-2, during R. rickettsii infection of endothelial cells. Steady-state levels for HO-1 mRNA were increased two- to threefold, as early as 4 h postinfection, whereas HO-2 mRNA was not affected. Induction of HO-1 mRNA was dependent on the dose of infection and occurred in a time-dependent manner, reaching maximal levels at 4 to 7 h. The increase in HO-1 mRNA occurred at the level of trancription as it was blocked by the transcriptional inhibitors, actinomycin D and alpha-amanitin. The eukaryotic protein synthesis inhibitor, cycloheximide, caused a >50% reduction in the infection-induced increase in HO-1 mRNA level, suggesting its dependence on de novo protein synthesis of host cell. The uptake of viable organisms appeared to be necessary, since inactivation of R. rickettsii by heat or formalin fixation, or incubation of cells with cytochalasin B to prevent entry resulted in marked inhibition of HO-1 response. N-Acetyl-L-cysteine, a known oxidant scavenger, inhibited the HO-1 induction by R. rickettsii. Finally, Western analysis with a specific monoclonal antibody revealed higher levels of HO-1 protein ( approximately 32 kDa), confirming that changes in HO-1 mRNA levels were followed by increases in the levels of protein. The findings indicate that R. rickettsii infection induces HO-1 expression in host endothelial cells and suggest an important role for this enzyme in cellular response to infection, possibly by serving a protective function against oxidative injury.