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Introduction: So far an adequate model for testing the effectiveness of drugs in infections of the urinary tract in-vivo has not been available. The present study was therefore undertaken with the aim of using experimental cystitis to develop a model of chronic bacteriuria by the use of Escherichia coli and Pseudomonas, to determine, by comparative histology, whether morphological differences do exist between the two types of infection, and to see whether the infections remain restricted to the urinary bladder in both cases. Material and Methods: Female albino rats weighing 200–250 g were used for the time serial tests. The anterior bladder wall was injured by puncture incision and suturing. A serotype O 4 : H 5 strain of Escherichia coli, and a strain of Pseudomonas pyocyanea cultured from the urine of patients were used for the infections. The number of germs/ml in the applied suspension was 1 × 10 9 in the case of Escherichia coli, and 7 × 10 8 in the case of Pseudomonas. The number of germs was determined microscopically, and the concentration of germs controlled by subsequent incubation and counting. Therapy was attempted with Carbenicillin and Gentamycin, and the results were compared with the bacteriological findings in untreated rats. Histological examination was performed on paraffin sections of formalin-fixed material subjected to H & E, PAS and van Gieson staining. Results and Discussion: Bacteriuria was produced by direct injection of Escherichia coli and Pseudomonas pyoceanea into the urinary bladder following bladder injury by puncture incision. Although diffuse chronic experimental cystitis could not be produced in either model, there was protracted significant bacteriuria, as shown by histological and bacteriological examination of the bladder and kidneys and bacteriological urinalysis. Acute suppurative inflammation following infection with Escherichia coli was localized in the injured portions of the anterior bladder wall where it was demonstrable for one to two weeks. Bacterioscopically the colonies of Escherichia coli were restricted to the injured mucosal portions of the anterior bladder wall, whereas Pseudomonas was found on the entire mucosa, regardless of previous injury. To produce diffuse suppurative inflammation Escherichia coli requires a previously injured mucosa as a point of entry. Pseudomonas, however, provides its point of entry into the submucosa by its exotoxin effect, thus creating the condition required for the occurrence of diffuse cystitis. In one third of the investigated animals renal affection was present in the form of pyelitis and ascending pyelonephritis, occurring within the first two weeks after infection with Escherichia coli, and from the second to sixth week after infection with Pseudomonas. The described model of bacteriuria has proved to be a suitable method for testing the effectiveness of antibiotics in infections of the urinary tract and bladder.