Active reabsorption of Na⁺ across the alveolar epithelium is essential for lung fluid homeostasis. Na⁺ entry at the apical membrane of the lung epithelial cell is predominantly via the amiloride–sensitive Na⁺ channel (ENaC) down its electrochemical gradient. This gradient is generated and maintained by Na⁺ extrusion via the Na⁺K⁺ATPase pump located at the basolateral membrane. Phenformin, an oral hypoglycaemic biguanide has been shown to influence cellular metabolism and membrane function. This has recently been attributed, in part, to its ability to activate AMP–activated protein kinase (AMPK) (Sakamoto et al. 2004). Therefore, we explored the effect of phenformin and activation of AMPK on ion transport across H441 human lung epithelial cells. H441 cells were cultured on permeable supports for 7 days and monolayers were treated both apically and basolaterally with 10mM phenformin for 1 hour. Monolayers were then mounted in Ussing chambers where phenformin was circulated in physiological saline throughout the course of the experiment. Spontaneous short circuit current (I_sc) was measured by clamping transepithelial voltage (V_t) at zero. Statistical analysis was carried out using Student’s t–test where p values of < 0.05 were considered significant. Data are presented as mean ± SEM Treatment with phenformin resulted in a significant decrease in spontaneous I_sc (18.9 ± 2.29 μA.cm^-2) compared to that of controls (42.3 ±1.8 μA.cm^-2, p < 0.001, n = 4). This was not the result of differences in the resistive properties between treated and untreated monolayers (p=0.33, n = 4). Forskolin (10μM) induced a rise in I_sc of 13.8 ± 4.2 μA.cm^-2 in control cells which was not evident in the presence of phenformin (0.2 ± 0.32 μA.cm^-2 p < 0.05, n = 3). Application of 10μM amiloride to the monolayers showed that phenformin significantly reduced the amiloride–sensitive component of I_sc (14.3 ± 1.3 μA.cm^-2) compared to control (46.9 ± 4.9 μA.cm^-2 p < 0.01, n = 3). Na⁺K⁺ATPase pump capacity was determined by apical permeabilisation with 75μM nystatin followed by blockade with 100μM ouabain. Ouabain-sensitive current was also significantly reduced in phenformin treated cells (12.3 ± 0.3 μA.cm^-2) compared to that of controls (39.4 ± 6.2 μA.cm^-2 p = 0.01, n = 3). Phenformin induced a 3.8-fold rise in AMPK activity in H441 cells from 0.5 ± 0.1 to 1.8 ± 0.2 nmol min^-1 mg^-1 (p = 0.001, n = 4). Taken together these data indicate that 10mM phenformin suppresses amiloride-sensitive Na⁺ transport across H441 cells via a pathway that includes activation of AMPK and inhibition of Na⁺K⁺ATPase.