Acute lung injury (ALI) is characterized by inflammatory disruption of the alveolar—vascular barrier, resulting in severe respiratory compromise. Inhibition of the intercellular messenger protein, Group V phospholipase A2 (gVPLA2), blocks vascular permeability caused by LPS both in vivo and in vitro. In this investigation we studied the mechanism by which recombinant gVPLA2increases permeability of cultured human pulmonary endothelial cells (EC). Exogenous gVPLA2(500 nM), a highly hydrolytic enzyme, caused a significant increase in EC permeability that began within minutes and persisted for >10 hours. However, the major hydrolysis products of gVPLA2(Lyso-PC, Lyso-PG, LPA, arachidonic acid) did not cause EC structural rearrangement or loss of barrier function at concentrations >10 μM. Higher concentrations (≥ 30 μM) of these membrane hydrolysis products caused some increased permeability but were associated with EC toxicity (measured by propidium iodide incorporation) that did not occur with barrier disruption by gVPLA2(500 nM). Pharmacologic inhibition of multiple intracellular signaling pathways induced by gVPLA2 activity (ERK, p38, PI3K, cytosolic gIVPLA2) also did not prevent EC barrier disruption by gVPLA2. Finally, pretreatment with heparinase to prevent internalization of gVPLA2 did not inhibit EC barrier disruption by gVPLA2. Our data thus indicate that gVPLA2 increases pulmonary EC permeability directly through action as a membrane hydrolytic agent. Disruption of EC barrier function does not depend upon membrane hydrolysis products, gVPLA2 internalization, or upregulation of downstream intracellular signaling.