Ontogenetic growth in the forelimb and hindlimb of the dinosaur Maiasaura peeblesorum (Ornithischia: Hadrosauridae) from the Upper Cretaceous (Campanian-Maastrichtian) Two Medicine Formation of Montana is investigated by multivariate and bivariate morphometrics and the biomechanics of beam theory. Results support a hypothesis of an age-dependent selection of stance. Juveniles walked primarily as bipeds. As an individual matured, its predominant stance shifted to quadrupedality. Within the forelimb, morphometric results show a probable allometric enlargement of postural muscles, an allometric increase in the lever arms of protractor muscles, and an increased robustness of the humerus to enhance its resistance to bending stresses. In contrast, the hindlimb is characterized by a relative decrease in the resistance of the femur and tibia to bending stresses. In addition, there is an allometric enlargement of the femoral fourth trochanter and positive allometry of the lengths of metatarsals III and IV. The most likely explanation for the different growth patterns is that the hindlimb was sufficiently robust at a young age to accommodate increased postural and locomotory stresses through largely isometric growth, whereas a behavioral shift to quadrupedality in older individuals necessitated an allometric response in the forelimb. Osteological adaptations for weight-bearing in the manus include metacarpals that are united firmly with a resultant cross-sectional shape that is resistant to bending and hyperextendable joints between the metacarpals and first phalanges. It is probable that flexor muscles that attached to the caudal surface of the metacarpals reinforced the reduced carpus and lessened the likelihood of collapse during quadrupedality.