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We have done experiments to determine the site(s) of resistance to the outward flow of lung fluid during apnoea and the inward flow of fluid during breathing in fetal lambs. Pressures were measured at various sites in the upper airway of ten chronically instrumented fetal lambs. Three fetuses were tracheostomized, two fetuses had nasopharyngeal tubes, one fetus had a tube through the body of its larynx only and four fetuses were studied with their upper airway intact. During apnoea, oropharyngeal pressure as well as tracheal pressure was greater than amniotic pressure. Tracheostomy or insertion of a nasopharyngeal tube (that extended from the amniotic cavity to the oropharynx) eliminated the standing pressure gradient between trachea and amniotic cavity. During normal breathing, negative phasic changes in oropharyngeal pressure occurred only when there was substantial posterior cricoarytenoid activity. Tracheostomy substantially attenuated phasic changes in tracheal pressure during breathing. From these data we conclude that the larynx provides a substantial resistance to the movement of fluid towards the lungs during normal breathing despite the presence of laryngeal abductor activity but does not offer a resistance to the secretion of lung fluid. The small positive tracheal pressure during apnoea most likely results from the secretion of lung fluid into a closed pharyngeal and buccal cavity or against a high nasal and oral resistance.

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