An acute exudative Escherichia coli pyelonephritis rat model was used to study the influence of progressive pyelonephritis on the efficacy of antibiotic treatment. In this model, transient ureteral obstruction after E. coli bladder inoculation induces early bacterial multiplication in the kidney parenchyma, and the bacterial counts peak by 48 h. The inflammatory response (assessed by the increase in kidney weight) is somewhat delayed, starting 36 h after inoculation and peaking by 72 h. Groups of rats received 4 doses over 48 h of saline, ceftriaxone (100 mg/kg), or ceftriaxone (100 mg/kg) plus gentamicin (4 mg/kg). These treatments were initiated 24, 36, 48, or 72 h after bladder inoculation. Antibiotic treatment started at 24 h was significantly more effective in reducing bacterial counts in the kidney parenchyma than at any later therapy onset. Only when started 24 h after inoculation was the synergistic combination of ceftriaxone plus gentamicin more effective in reducing bacterial counts than ceftriaxone alone. Ceftriaxone and ceftriaxone plus gentamicin regimens started at 24 h reduced significantly (by 42 and 55%, respectively) the incidence of acute exudative pyelonephritis when compared with the incidence in saline-treated controls. Early therapy onset (24 h) strikingly reduced the development of the inflammatory response. This reduction was less marked when antibiotic therapy was started at 36 h and no longer apparent when therapy onset was delayed up to 48 or 72 h. In conclusion, the efficacy of antibiotics in eradicating bacteria from the kidney parenchyma and in preventing acute exudative pyelonephritis was markedly hampered by the development of pyelonephritis.