A sinusoidal flow generator was connected to adult endotracheal tubes of sizes 5-10 and was used to simulate human ventilation. Measurement of the changes in pressure and flow allowed calculation of the work imposed on breathing by endotracheal tubes. The work of breathing increased with increasing ventilatory rate, tidal volume, and decreasing tube diameter. We discuss the suggestion that the work imposed on breathing by an endotracheal tube is a more appropriate unit for comparison than resistance to air flow.