The direct structural, metabolic, and physiologic effects of Escherichia coli endotoxin on bovine pulmonary endothelial monolayers and on the intact endothelial layer of bovine pulmonary artery intimal explants were examined. Endothelial monolayers exposed to E coli endotoxin (0.001, 0.01, 0.1, 1.0, and 10 micrograms/ml) for 24 hours in the absence of bovine fetal calf serum (FCS) showed a dose-dependent response, as demonstrated by number of pyknotic cells and lactate dehydrogenase release that was enhanced by addition of FCS. Prostacyclin production was increased only in the presence of FCS. Endotoxin also caused an increase in permeability. Endothelial cells on nitrocellulose filters placed in chemotaxis chambers with radioactive tracers in the upper well showed a significant 25% increase in rate of equilibration (counts in lower well/counts in upper well) of 3H-water after 2 and 3 hours' incubation with endotoxin (3 hours' endotoxin = 0.89 +/- 0.03 m +/- SE; no endotoxin = 0.69 +/- 0.05) and a 40% increase in equilibration of 125I-albumin at three hours (3 hours' endotoxin = 0.40 +/- 0.03; no endotoxin = 0.27 +/- 0.02). An increase in hydraulic conductance was also seen at 1 hour. Electron microscopy of the endothelial layer of intimal explants showed dilatations in the intercellular junctions and cellular changes representing contraction--increased prominence of cytoplasmic filaments, nuclear crenation, and cytoplasmic protrusions--at 30 and 60 minutes. From 2 hours evidence of cell death was found. Thus, endotoxin causes structural and metabolic changes in pulmonary endothelial cells and an increase in permeability of the endothelial layer. The injury occurs in the absence of FCS but is enhanced by its addition.