The dynamics of endoneurial edema were studied by quantifying endoneurial fluid pressure (EFP) during the development of lead neuropathy and correlating these data with changes in blood-nerve barrier permeability and with morphological alterations in nerves, capillaries, and Schwann cells. EFP measured from the sciatic nerve in control Long-Evans rats was 2.1 +/- 1.0 cm H2O. EFP was significantly elevated 7 weeks after animals were started on a diet containing 6% lead carbonate, and it increased progressively until a plateau in pressure was reached between weeks 9 and 11. Thereafter, EFP gradually returned to normal values. The progressive increase in EFP was highly correlated with the extravasation of osmotically active macromolecules, traced by fluorescein isothiocyanate-dextran compounds of graded molecular weight and by horseradish peroxidase (HRP). Electron microscopy revealed extravasation of HRP between endothelial cells, intranuclear inclusions characteristic of lead poisoning in Schwann cell nuclei, demyelination, and remyelination. The observation of intranuclear inclusions consistent with lead deposition in Schwann cells strengthens the hypothesis that extravasated lead in the interstitial fluid causes direct injury to Schwann cells, giving rise to demyelination. Nerve compliance was determined.