The effects of changing body posture on breathing frequency (f) of anesthetized newborn rats, rabbits, and dogs have been studied. No significant changes were observed between horizontal postures (supine, prone, left, and right lateral). In the vertical head-down position, most of the animals did not show any significant variation of f. Breathing rate dropped significantly in the head-up position, and in all the animals, it was still below the horizontal value 1 min after the vertical head-up tilting. After bilateral vagotomy, no significant change with body posture occurred. In unanesthetized newborns, the +45 degree head-up tilting only occasionally caused a drop of f and to a lesser extent than after anesthesia. By comparing these results with those reported in adults, it is concluded that the postural effects of f are less evident in the newborn than in the adult. Even if a quantitative difference in the mechanical events after the change in body posture is the most likely explanation of the lower sensitivity of the newborn, other nonmechanical factors could also be involved.