Vital capacity and its subdivisions, alveolar gas composition, single breath nitrogen wash-out and the forced expiratory flow-volume curves were measured in 71 young persons (mean age 19.7 years) in sitting and supine positions. A decreased expiratory reserve volume accompanied by a tendency to an increased slope of the alveolar N2 plateau and an increased closing volume, were observed in supine position. Forced expiratory flow indices (PEF, FEF50, FEF75–85) were lower in supine position suggesting increased respiratory resistance. Alveolar CO2 partial pressure was higher in recumbency than in upright position. The increased slope of the alveolar N2 plateau and changes in alveolar gas composition in recumbency are considered to indicate changes in the distribution of inspired air and in the ventilation-perfusion relationship.