Our objective was to evaluate experimental induction of empyema in rats by intrapleural inoculation of Staphylococcus aureus by means of thoracentesis with pleural pressure monitoring. Forty female albino Wistar rats, anesthetized with droperidol and fentanyl, underwent intrapleural inoculation of a 0.2-ml solution. Group I ( N=25) received 10(10) colony-forming units/ml of Staphylococcus aureus cultivated in brain-heart infusion agar (BHI); group II (N = 15), the control group, received sterile BHI agar. Rats were inoculated after right hemithorax thoracentesis with a needle connected to an oscilloscope for pressure monitoring to confirm that the needle was inside the pleural space. Animals were killed after 3 (groups Ia and IIa) to 5 (groups Ib and IIb) days with sodium pentobarbital. The macroscopic changes, amount of pleural fluid, and anatomopathological aspects of pleura and lungs were recorded, as well as death causes and bacteriological findings of pleural fluid for animals that died before the time established for euthanasia. In group I, three animals died of thoracentesis complications, and five others died in the first 24 h due to septicemia; blood and spleen cultures isolated the bacteria previously inoculated. In group II, there was one death of unknown cause. Of the 17 rats inoculated with bacteria, nine (group Ia) were killed on the 3rd day; all had bacteria in pleural fluid (volume 0.5-3.8 ml). The other eight rats (group Ib) were killed on the 5th day; three (39.5%) had pleural fluid with bacteria (0.5-1.5 ml), and five (60.5%) had no pleural fluid. Rats from group II killed on the 3rd (group IIa) and 5th (group IIb) days had no pleural fluid. Pathologic examination revealed inflammatory infiltrate (93.75%) and fibrin (18.75%) in pleura, and inflammatory interstitial infiltrate (12.5%) in the right lung in group I; no changes were observed in 93.3% of the lungs in group II. Macroscopic examination revealed only turbid and bloody pleural fluid (class I) without pleural adhesions. Pleural inflammatory infiltrate was found in rats that received the bacteria but had no fluid at necropsy (class 0). One control rat, although with no clinical signs of disease or pleural fluid, had signs of pleural and pulmonary infection at necropsy. We conclude that empyema may be induced in rats by the inoculation of Staphylococcus aureus by means of thoracentesis with pleural pressure monitoring. The highest amount of pleural fluid was observed 3 days after bacterial inoculation.