It is widely believed that infants are obligatory nasal breathers. We studied 19 infants, 1 to 230 days of age, for respiratory response to acute nasal occlusion. Lips were kept apart. Oropharyngeal structures were monitored by fluoroscopy, whereas respiratory movements and oral flow were recorded. We systematically observed before and during nasal occlusion tight apposition of the soft palate and the tongue, closing the oropharyngeal isthmus. After a variable time (mean 7.8 s, range 0.6 to 32 s), the soft palate rose and oral breathing was initiated. Time required to mouth-breathe was related to age and/or conscious state, older and/or awake infants responding faster than younger and/or asleep infants. In 9 others, when nasal occlusion was performed with the mouth closed, results were comparable to those obtained in infants with mouths open. In 3 infants, electroencephalograph (EEG) records showed quiet non-REM sleep. Nasal occlusion resulted in an immediate arousal reaction, followed after a variable time by mouth breathing. We conclude that infants are not obligatory nasal breathers. They can breathe through the mouth by detaching the soft palate from the tongue, thus opening the oropharyngeal isthmus.