Ventilation is reduced in Lmx1bf/f/p mice during postnatal development. A–E, Data are presented from two groups of mice: one studied using flow-through plethysmography at 27°C (A, B) and another studied using stop-flow plethysmography at 24 and 30°C (C–E). A, Ventilation, which was normalized to weight and expressed relative to WT mice (black; n = 8–14 animals per data point), was lower in Lmx1bf/f/p mice (blue; n = 5–13) up to the age of P28 (*p ≤ 0.02), but was normal in adults. B, Similarly, the ratio of ventilation to oxygen consumption [calculated in ml/min in this case (see Materials and Methods)] was also reduced in Lmx1bf/f/p mice (n = 5–6) relative to WT mice (n = 8–9) from P20–P28 mice but was normal in adults. *p ≤ 0.045. C, Ventilation was also reduced (p ≤ 0.015) in Lmx1bf/f/p mice (n = 6–12) compared with WT mice (n = 7–10) up to P6 at a warm ambient temperature (30°C; open symbols), and up to P12 at 24°C (p ≤ 0.014; solid symbols). D, Breath amplitude (normalized to weight and expressed as a percentage of WT value) was lower in Lmx1bf/f/p mice up to P4 at both temperatures (p ≤ 0.006) and up to P9 at 24°C (p < 0.001). E, Breathing frequency was also lower in Lmx1bf/f/p mice up to P9 at both temperatures (p ≤ 0.004) and up to P12 at 24°C (p < 0.001). *Difference between genotypes at 24°C; **difference between genotypes at both temperatures. Data points were shifted along the x-axis by ±0.2 d of age in C and D for clarity. Adult data shown here are reproduced from an earlier publication for comparison (). F, Breathing was more irregular in Lmx1bf/f/p mice relative to WT mice (**p < 0.05) from P2 to P6, as quantified by the CV IBI when measured at ambient temperatures of 24°C (solid symbols) and 30°C (open symbols). G, Animal temperature was slightly lower in Lmx1bf/f/p mice relative to WT mice at an ambient temperature of 27°C at the ages of P12, P20, and P28 (*p ≤ 0.049) but was not significantly different at other ages, including adults.