The present study was designed to determine the aerobic and anaerobic energy contributions to 400-m Flat (400 mF) and 400-m hurdles (400 mH) using the accumulated oxygen deficit method. Six nationally ranked athletes, specializing in 400 mH and familiar with 400 mF volunteered to participate in this study. All the participants performed 3 track-running sessions. The first session determine the maximal oxygen uptake and maximal aerobic speed using lightweight ambulatory respiratory gas exchange measurements (o2000, Medical Graphics). The second and third tests consist of a 400 mF and a 400 mH performed on the track in a randomized counterbalanced order. Accumulated oxygen deficit determined during the 400 mF was significantly higher than that determined during the 400 mH (65.0 +/- 10.0 mlxkg vs. 44.1 +/- 7.4 mlxkg, p < 0.05). Thus, the aerobic contribution calculated was significantly higher during the 400 mH compared to during the 400 mF (43.0 +/- 2.0 vs. 37.4 +/- 2.7%, p < 0.05, respectively). These results strongly suggest that the aerobic contribution is greater during a 400 mH compared to during a 400 mF. Thus, this study provides a scientific rationale behind the coaches' practice and contributes to a better understanding of the differences between 400 mF and 400 mH. Then, the coaches must propose different training programs for both 400 mF and 400 mH runners.