The resitances to air flow in 3 sizes of endotracheal tubes and connectors were measured and compared with the reported upper airway resistance of the infant. Three and 3.5 mm I.D. [internal diameter] tubes of the usual length appeared to impose little chance of increasing the work of breathing in the spontaneously-breathing newborn infant. Compared with the infant''s airway resistance 2.5 mm I.D. tubes had a high resistance and should be used only in association with artificial ventilation.