OBJECTIVE To establish optimal exposure parameters for lateral oblique radiographs of rat mandibles using an intraoral X-ray machine. METHODS A positioning apparatus, previously tested for its reproducibility, was used to obtain radiographs of four heads of formaldehyde-preserved Wistar rats. Radiographs were exposed at 50 kV and 8 mA using four size 2 films (Insight, Ektaspeed Plus, Ultraspeed and D-Speed; Eastman Kodak Co., Rochester, NY), two focal distances (30 cm and 60 cm) and seven exposure times (0.6 s, 0.7 s, 0.8 s, 1.0 s, 1.5 s, 2.0 s and 2.5 s). The radiographs were evaluated by two examiners and scored for image quality: 1, very poor; 2, poor; 3, fair; 4, good; and 5, excellent. The evaluations were repeated at 30 days. RESULTS Intraobserver reproducibility was substantial (kappa = 0.798 and 0.667). Also, the two examiners showed substantial agreement (kappa = 0.726). There was a significant difference (alpha = 1%) between the scores for the films under study. At a 30 cm distance, E-speed films had mean scores of 4 and 5 at 0.8 s and 1 s exposure times, and D-speed films, at 1.5 s, 2 s and 2.5 s exposure times. At a 60 cm distance, the best results were found for E-speed films at 2.0 s and 2.5 s exposure times. CONCLUSION The films under study did not show any significant difference in the quality of rat mandible images as long as exposure times were adequate to their sensitivity and focal distance.