1. A new method for measuring static compliance, which does not require general anaesthesia or a leak-free airway, is described for infants and young children. 2. Thoracic volume changes produced by static inflation pressures are measured by a respiratory jacket, and results compare well with those obtained by the syringe-inflation method. 3. This method can be used in the intensive care of sick infants, where it allows a new approach to the investigation of lung mechanics.