Objective To study the mechanisms responsible for the changes in alveolar fluid clearence in rats with malnutrition． Methods Rats were fasted but allowed access to water for 120 hours． Alveolar fluid clearance ( AFC) ，lung water content ( TLW) and extravascular lung water ( EVLW) of the rats were calculated in the rats with malnutrition．Isotonic 5% album in solutions with amiloride，ouabain and terbutalin were instilled into the distal airways in the malnutrition and control rat lungs，and the AFC was examined． Results As compared with the control rats ( 18. 5 ±2. 21% ) ，AFC in the rats with malnutrition for 48 h was not decreased ( 19. 7 ± 3. 19% ) ． AFC in the rats under conditions of malnutrition for 120 h was decreased ( 9. 50 ± 2. 19% ) ． Amiloride and ouabain could reduce the AFC in both malnutrition and control rats． There was no significant difference in the magnitude of the terbutalin-stimilated AFC in rats under conditions of malnutrition compared with that in control rats． Conclusion Malnutrition primarily impairs amiloridesensitive and amiloride-insensitive alveolar fluid clearance，leading to pulmonary edema．