To investigate the biomechanical effects of juvenile growth changes in the rabbit masticatory apparatus a comparison was made of mastication in just-weaned and adult animals. Mandibular movements in two planes were registered by cineradiography. Masticatory muscle activity was recorded by finewire electromyography. The same pattern of unilateral mastication was present in the two ages. The most important changes in the jaw movements are 1) a decrease of jaw opening speed and chewing frequency and an increase in jaw opening time, 2) a decrease in maximum gape in soft food and an unaltered gape in small-particle hard food, and 3) an increase in lateral jaw excursion, mainly due to a more pronounced movement of the jaw to the balancing side (lingual phase). The contraction patterns were basically similar in the two ages. The higher chewing frequency in young animals was attained by a larger degree of overlap between opening and closing muscle activities. Young animals used relatively more EMG activity to chew hay, the hardest food. The changes in opening speed, gape, and chewing frequency are consistent with earlier predictions from the morphological changes, and so is the extra activity needed to chew hard food. The increase in lateral excursion was not predicted. It is suggested to be caused by cheek teeth wear, making possible smooth occlusal guidance of the jaw at the balancing side. Some of the changes in juvenile morphology can be viewed as adaptations to a changing diet.