In vivo cinemicroscopic studies of subpleural alveoli were conducted in dogs for 4 h after pulmonary lavage with 5% Tween 20 and after lavage with normal saline. Saline-lavaged alveoli showed little change in alveolar size during tidal ventilation, whereas Tween lavage resulted both in alveolar recruitment and marked variation in alveolar size from end expiration to peak inspiration, with total collapse frequently occurring by end expiration. Following Tween, alveolar stability decreased, but alveolar capillary perfusion increased. Marked recovery of stability by 4 h was noted in most alveoli. Wilhelmy balance studies on lung extracts showed a decrease in surfactant function 30 min after Tween and a partial recovery of surfactant activity after 4 h. This study provides in vivo evidence that normal surfactant function is critical to alveolar stability and that alveolar stability is markedly restored 4 h after acute deactivation or displacement by Tween. Surfactant deactivation and loss of alveolar stability are associated with increased alveolar capillary perfusion creating significant ventilation-perfusion ratio abnormalities.