Tumor necrosis factor (TNF)-α is released in acute inflammatory lung syndromes linked to the extensive vascular dysfunction associated with increased permeability and endothelial cell apoptosis. TNF-α induced significant decreases in transcellular electrical resistance across pulmonary endothelial cell monolayers, reflecting vascular barrier dysfunction (beginning at 4 h and persisting for 48 h). TNF-α also triggered endothelial cell apoptosis beginning at 4 h, which was attenuated by the caspase inhibitorZ-Val-Ala-Asp-fluoromethylketone. Exploring the involvement of the actomyosin cytoskeleton in these important endothelial cell responses, we determined that TNF-α significantly increased myosin light chain (MLC) phosphorylation, with prominent stress fiber and paracellular gap formation, which paralleled the onset of decreases in transcellular electrical resistance and enhanced apoptosis. Reductions in MLC phosphorylation by the inhibition of either MLC kinase (ML-7, cholera toxin) or Rho kinase (Y-27632) dramatically attenuated TNF-α-induced stress fiber formation, indexes of apoptosis, and caspase-8 activity but not TNF-α-induced barrier dysfunction. These studies indicate a central role for the endothelial cell cytoskeleton in TNF-α-mediated apoptosis, whereas TNF-α-induced vascular permeability appears to evolve independently of contractile tension generation.