Introduction: Inappropriate mechanical ventilation in patients with acute respiratory distress syndrome can lead to ventilator induced lung injury (VILI) and increase the morbidity and mortality. Reopening collapsed lung units may significantly reduce VILI, but the mechanisms governing lung recruitment are unclear. We thus investigated the dynamics of lung recruitment at the alveolar level. Methods: Rats (n=6) were anesthetized and mechanically ventilated. The lungs were then lavaged with saline to simulate ARDS. A left thoracotomy was performed, and an in vivo microscope placed on the lung surface. The lung was recruited to three recruitment pressures (RP) of 20, 30 or 40 cmH₂O for 40 seconds while subpleural alveoli were continuously filmed. Following measurement of microscopic alveolar recruitment, the lungs were excised and macroscopic gross lung recruitment was digitally filmed. Recruitment was quantified by computer image analysis, and data interpreted using a mathematical model. Results: The majority of alveolar recruitment 78.3 ±7.4% and 84.6% ±5.1% occurred in the first two seconds (T2) following application of RP 30 and 40, respectively. Only 51.9±5.4 % of the microscopic field was recruited by T2 with RP 20. There was limited recruitment from T2 to T40 at all RPs. The majority of gross lung recruitment also occurred by T2 with gradual recruitment to T40. The data were accurately predicted by a mathematical model incorporating the effects of both pressure and time. Conclusions: Alveolar recruitment is determined by the magnitude of recruiting pressure and length of time pressure is applied, a concept supported by our mathematical model. Such a temporal dependence of alveolar recruitment needs to be considered when designing recruitment maneuvers for clinical application.