Conditioned media from two human malignant gliomas, C6 rat glioma, Walker 256 carcinosarcoma, and normal human glia were concentrated 50-fold to create a culture supernatant (SUP-C). The effect of SUP-C on rat brain capillary permeability was investigated by measuring the entry of 14C-aminoisobutyric acid (14C-AIB) by means of quantitative autoradiography. The SUP-C contained proteins with a molecular weight of 10 kD or greater. The SUP-C from all tumor cells markedly increased brain capillary permeability, indicating the presence of a permeability factor, whereas that from normal glial cells did not. Glioma cells produced more factor after incubation for 20 hours than 4 hours. The activity of capillary permeability factor in the SUP-C was inhibited by pretreatment of animals with BW755C (lipoxygenase inhibitor), but not with indomethacin (cyclo-oxygenase inhibitor). Pretreatment of animals with dexamethasone prior to intracerebral infusion of tumor SUP-C significantly reduced the factor-induced increase in capillary permeability. On the other hand, coincubating glioma cells with dexamethasone produced SUP-C with a permeability activity that was about one and a half times greater than that without dexamethasone. These results indicate that glucocorticoids produce their anti-edema effects by directly acting on capillary endothelial cells, possibly through the inhibition of phospholipase A2 activity, resulting in a decrease of lipoxygenase rather than cyclo-oxygenase products. The production of capillary permeability factor by tumor cells was not inhibited, but rather enhanced, by administration of glucocorticoids.