Pulmonary endothelial cell (EC) barrier dysfunction and recovery is critical to the pathophysiology of acute respiratory distress syndrome. Cytoskeletal and subsequent cell membrane dynamics play a key mechanistic role in determination of EC barrier integrity. Here, we characterizAQe the actin related protein 2/3 (Arp 2/3) complex, a regulator of peripheral branched actin polymerization, in human pulmonary EC barrier function through studies of transendothelial electrical resistance (TER), intercellular gap formation, peripheral cytoskeletal structures and lamellipodia. Compared to control, Arp 2/3 inhibition with the small molecule inhibitor CK-666 results in a reduction of baseline barrier function (1,241 +/- 53 vs 988 +/- 64 ohm; p < 0.01), S1P-induced barrier enhancement and delayed recovery of barrier function after thrombin (143 +/- 14 vs 93 +/- 6 min; p < 0.01). Functional changes of Arp 2/3 inhibition on barrier integrity are associated temporally with increased intercellular gap area at baseline (0.456 +/- 0.02 vs 0.299 +/- 0.02; p < 0.05) and thirty minutes after thrombin (0.885 +/- 0.03 vs 0.754 +/- 0.03; p < 0.05). Immunofluorescent microscopy reveals reduced lamellipodia formation after S1P and during thrombin recovery in Arp 2/3 inhibited cells. Individual lamellipodia demonstrate reduced depth following Arp 2/3 inhibition vs vehicle at baseline (1.83 +/- 0.41 vs 2.55 +/- 0.46 mm; p < 0.05) and thirty minutes after S1P treatment (1.53 +/- 0.37 vs 2.09 +/- 0.36 mm; p < 0.05). These results establish a critical role for Arp 2/3 activity in determination of pulmonary endothelial barrier function and recovery through formation of EC lamellipodia and closure of intercellular gaps.