Abstract Purpose: To evaluate the concentration of transforming growth factor beta 1 (TGFB1) levels in a rat pleural effusion obtained by inoculation of intrapleural bacteria or turpentine through thoracentesis. Methods: Thirty-Nine Wistar rats were divided into three groups: Staphylococcus aureus (SA, n = 17); Streptococcus pneumoniae (SP, n = 12); and turpentine (control, n = 10). Pleural fluid was collected through ultrasound-guided thoracentesis 12 h, 24 h, and 36 h after instillation of bacteria or turpentine. Levels of TGFB1 were measured in pleural fluid. Results: At 12 h, mean TGFB1concentrations were 5.3450 pg/mL in the SA group, 5.3449 pg/mL in the SP group, and 5.3450 pg/mL in controls. At 24 h, they were 4.6700 pg/mL in the SA group, 4.6700 pg/mL in the SP group, and 4.6700 pg/mL in controls. At 36 h, they were 4.6699 pg/mL in the SA group and in control. No difference was observed among the groups in mean TGFB1concentration (p = 0.12); however, a significant intragroup reduction in mean TGFB1 was observed between 12 and 24 h (p < 0.01). Conclusion: The transforming growth factor beta 1 concentrations were not useful as a diagnostic tool or an early marker of infected pleural effusion.