A new method of measuring the static compliance of the respiratory system in the awake, non-paralysed infant is described. The static compliance was measured by this method in 12 healthy premature infants aged from 1 day to 30 weeks. In addition, the dynamic compliance was measured by the oesophageal balloon technique. The static compliance was significantly higher than the dynamic, and this difference was related to the respiratory rate. These infants therefore demonstrated frequency dependence of dynamic compliance.