Acute obstructive pyelonephritis (PN) caused by Peptococcus niger (Pn), E. coli and mixed infection (Pn + E. coli) was modeled in rabbits. Morphologic examination showed that in animals with peptococcal infection renal lesions were identical to those in escherichial PN. Hemorrhagic infarctions and vasculitis develop in animals with mixed infection. These morphological findings were confirmed bacteriologically. The experimental trial confirms the role of Pn in development of acute obstructive PN in animals.