A new model of experimental endocarditis, using electrocoagulation of native aortic valves, was used for the study of Q fever endocarditis. In the 20 guinea pigs electrocoagulated and inoculated with Coxiella burnetii Nine Mile phase I strain, 10 presented with infective endocarditis. Of these, 7 died spontaneously. All guinea pigs with endocarditis presented with blood cultures positive for C. burnetii, and C. burnetii antigen was found in their cardiac valves. Positive blood cultures or valvular immunopositive cells were not identified in either nonelectrocoagulated or noninoculated controls. This experimental model demonstrates that Q fever in an animal with previously damaged valves results in endocarditis. It could provide a new tool for the investigation of pathophysiology and antibiotic therapy for Q fever endocarditis.