Gas exchange and respiratory mechanics were studied during hip arthroplasty, according to Charnley(1961). Dead space measurements were made by capnography and thoracic compliance and resistance, calculated liom tracheal pressure variations and respiratory gas flow. PaO2fell by 2.5 kPa with the insertion of each prosthesis. Small increases in PaO2were also seen. Distal (approximately corresponding to alveolar) dead space increased by 25% when each prosthesis was anchored. Dynamic thoracic compliance fell by 30–40 ml/kPa at the insertion of either prosthesis, while inspiratory and expiratory resistances were unaffected. The findings may indicate interference with the functions of both the bronchial and the pulmonary vascular tre