Breathing during the first postnatal hours has not been examined in mice, the preferred mammalian species for genetic studies. We used whole body plethysmography to measure ventilation (VE), breath duration (T(TOT)), and tidal volume (VT) in mice delivered vaginally (VD) or by cesarean section (CS). In experiment 1, 101 VD and 100 CS pups aged 1, 6, 12, 24, or 48 h were exposed to 8% CO2 or 10% O2 for 90 s. In experiment 2, 31 VD pups aged 1, 12, or 24 h were exposed to 10% O2 for 5 min. Baseline breathing maturation was delayed in CS pups, but VE responses to hypercapnia and hypoxia were not significantly different between VD and CS pups [at postnatal age of 1 h (H1): 48 +/- 44 and 18 +/- 32%, respectively, in VD and CS pups combined]. The VE increase induced by hypoxia was greater at H12 (46 +/- 27%) because of T(TOT) response maturation. At all ages, hypoxic decline was ascribable mainly to a VT decrease, and posthypoxic decline was ascribable to a T(TOT) increase with apneas, suggesting different underlying neuronal mechanisms.