The aim of the present investigation is the complex study of experimental infection in pregnant ewes by means of clinical, serological, biological, histological and Electron microscopy methods. Four ewes, pregnant from the 2nd to 5th month, were infected by intravenous (in one case by intraperitoneal) routes with a C. burnetii strain at 10(6) ID 50/ml. The clinical illness in all of the animals was characterized by fever and two-phase temperature reaction on the 5th and 12th days. The clinical symptoms were as follows: torpidity, reduced appetite, thirst, conjuctivitis, rhinitis, rapid breathing. As a result of the developed latent infection, after the acute stage, the animals gave birth to three unviable lambs who died within 24 h. Another lamb was still-born. The lambs showed cachexia, arthritis, ataxia, wrinkled skin. The highest CF-titers (1:256-1:512) were reached on the 40th day, but serum antibodies (1:8-1:32) first appeared on the 8th day. The titers began to decrease on the 60th day. The pathomorphological changes testify to a latent infection characterized by placentitis, lymphocellular proliferation of the lamb's parenchymal organs and lymph nodes, multiple thromboses, interstitial pneumonia and plural proliferative changes. The EM exam showed rickettsiae in placentas mainly in the form of inclusions in cytoplasm of leukocytes and epithelial cells.