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Positive pressure mechanical ventilation is an essential treatment for acute respiratory distress syndrome (ARDS). However, this treatment may be accompanied by ventilator-associated lung injury, which can be avoided by early weaning. This requires an index that predicts the success of weaning. From the viewpoint of weaning, identifying the energy conservation rate of the lungs is meaningful in patients with ARDS. This study aimed to obtain the basic findings on whether the energy conservation rate can be an indicator of weaning in patients with ARDS. Four healthy adult males with no history of smoking participated in the experiment. Voluntary spontaneous breathing was performed at 5, 10, 15, 20, 25, 30, 35, 40, 45, and 50 cycles/min. Pressure, volume, and flow were measured using a ventilator, and the mechanical energy, mechanical work, and energy conservation rate of the lungs were calculated. The energy conservation rate increased significantly from 30 cycles/min in tachypnea. No significant difference was observed in the energy conservation rate between 20 cycles/min and 25 cycles/min. However, a significant difference was observed between 25 cycles/min and 30 cycles/min at 5％ significance level. The energy conservation rate per breath at 30 cycles/min was 26.4± 2.9％, and the energy conservation rate during inspiration at 30 cycles/min was 41.6± 4.9％. Given that the breathing frequency in the criteria of successful spontaneous breathing trials was less than 30 cycles/min and the energy conservation rate increased significantly to 26.4 ± 2.9％(breathing period) and to 41.6 ± 4.9％(inspiratory period) at 30 cycles/min. These values were predicted to be the threshold of weaning for patients with ARDS. The basic findings of this study indicate that the energy conservation rate may be an indicator of weaning in patients with ARDS. [ABSTRACT FROM AUTHOR]