We examined developmental changes in breathing pattern and the ventilatory response to hypoxia (7.4% O₂) in unanesthetized Swiss CD-1 mice ranging in age from postnatal day 0 to 42(P₀–P₄₂) using head-out plethysmography. The breathing pattern of P₀ mice was unstable. Apneas were frequent at P₀ (occupying 29 ± 6% of total time) but rare by P₃ (5 ± 2% of total time). Tidal volume increased in proportion to body mass (∼10–13 ml/kg), but increases in respiratory frequency (f) (55 ± 7, 130 ± 13, and 207 ± 20 cycles/min for P₀, P₃, and P₄₂, respectively) were responsible for developmental increases in minute ventilation (690 ± 90, 1,530 ± 250, and 2,170 ± 430 ml ⋅ min^{− 1} ⋅ kg^{− 1}for P₀, P₃, and P₄₂, respectively). Between P₀ and P₃, increases in f were mediated by reductions in apnea and inspiratory and expiratory times; beyond P₃, increases were due to reductions in expiratory time. Mice of all ages showed a biphasic hypoxic ventilatory response, which differed in two respects from the response typical of most mammals. First, the initial hyperpnea, which was greatest in mature animals, decreased developmentally from a maximum, relative to control, of 2.58 ± 0.29 in P₀ mice to 1.32 ± 0.09 in P₄₂mice. Second, whereas ventilation typically falls to or below control in most neonatal mammals, ventilation remained elevated relative to control throughout the hypoxic exposure in P₀ (1.73 ± 0.31), P₃ (1.64 ± 0.29), and P₉ (1.34 ± 0.17) mice but not in P₁₉ or P₄₂ mice.