Factor analysis of the steady state breathing cycle using Kr-81m gas wa performed in 7 normal volunteers. Two-factor analysis showed diaphragmatic and thoracic components, including the large airway space component. The contribution of the former was 19.2%±2.1% SD, and the contribution of the latter was 80.2%±2.1% SD. Three-factor analysis showed diaphragmatic (14.3% ±3.3% SD), thoracic (55.7% ±2.1% SD) and large airway space (29.5% ±12.1% SD) components. Factor analysis of dynamic structures appears to be a very useful method for the study of lung ventilation.