Thirteen children (mean age, 45 months) with nocturnal symptoms of upper airway obstruction, the result of enlarged tonsils, were tested during wakefulness (W) and sleep (S) induced by chloral hydrate (≤50 mg/kg). During W, lung mechanics, blood gas, breathing pattern, and airflows during tidal breathing were in the normal range. During S, total lung resistance increased significantly, and dynamic lung compliance and transcutaneous P02 decreased significantly. During S, the tidal volume (VT) and the mean inspiratory flow, normalized for body weight (BW), decreased whereas the ratio of the inspiratory time (T1) over the total duration of the respiratory cycle (TTOT) rose, indicating a longer contraction time of the respiratory muscles. The time to reach peak inspiratory flow, measured as a percentage of T1 (dT1/T1), increased in seven children, with no change in the ratio of the expiratory flow over the inspiratory flow, both measured at 50% of VT (EF50/IF50). In three other patients dT1/T1 decreased with an increase in EFsofIFso. We conclude that in children with enlarged tonsils, S modified lung mechanics, gas exchange, and the inspiratory components of the breathing pattern and airflow.