Endotoxic shock is associated with acute vascular endothelial injury resulting in edema. Tumor necrosis factor (TNF) and interleukin 1 (IL-1) are cytokines produced by endotoxin-stimulated mononuclear phagocytes that are potential mediators of endotoxic shock. In this study, we investigated the effects of TNF and IL-1 alpha on vascular endothelial cell permeability in vitro. The movement of radiolabeled macromolecules of different sizes (57Co-vitamin B12, 125I-cytochrome c, and 131I-albumin; 6.5-35A) across bovine aortic endothelial cell monolayers was measured after exposure to these cytokines. TNF induced a time- and dose-dependent increase in endothelial cell monolayer permeability that was enhanced in the presence of serum. The peak increase was noted after 12 h of incubation with less alteration of permeability with longer incubations. IL-1 alpha caused a similar time-dependent increase in endothelial cell monolayer permeability, but the peak effect of IL-1 alpha was seen after 24 h. Therefore the increased permeability seen with TNF cannot be explained by release of endogenous IL-1 alone. Neither TNF nor IL-1 alpha increased release of [14C]adenine, and the only effect on lactate dehydrogenase release was a small, but statistically significant, increase after 24 h of incubation. From these studies, we conclude that TNF and IL-1 alpha directly increase vascular endothelial cell permeability in vitro and speculate that these cytokines may be involved in the acute vascular endothelial injury associated with endotoxic shock.