The mara Dolichotis patagonum (Caviomorpha, Caviidae) is probably the extant rodent with the most pronounced postcranial specializations for fast locomotion. When running the species can reach a speed of ~40 km/h. It has been suggested that its body posture and limb bones show evolutionary parallelism regarding small-sized artiodactyl species. Due to its elongated limbs and relatively large body mass (average 8 kg) compared to other rodents, its limb bones may experience large loads at high speeds. Using kinematic data from high speed video films and skeletal dimensions of museum specimens, ground reaction forces and stresses acting on the humerus and tibia during different gaits were estimated. Values of bending stress obtained for the tibia were greater than those for the humerus during walking, gallop and “pronk”. The stress experienced by the tibia when running doubled the value obtained when walking (54.5 vs. 26.1 MPa, respectively). Estimated bone safety factors were less than half during fast locomotion (3.1) compared to walking (6.4). It is discussed how maras’ body posture and skeletal dimensions affect bone stress experienced during locomotion.