1,3,5-Trinitrobenzene (TNB) is a soil and water contaminant at certain military installations. Encephalopathy in rats given 10 daily oral doses of TNB has been reported. The lesion was bilaterally symmetric vacuolation and microcavitation in the cerebellar roof nuclei, vestibular nuclei, olivary nuclei, and inferior colliculi. The contribution of the blood-brain barrier (BBB) in the genesis of these lesions remains uncertain. One of the main goals of the present work was to evaluate the functional state of the BBB. Male Fischer 344 rats (five rats/group) were euthanatized after four, five, six, seven, eight, or 10 daily doses of TNB (71 mg/kg). A different set of rats (five rats/group) was allowed to recover for 10 or 30 days after receiving 10 doses of TNB. Integrity of the BBB was assessed by immunohistochemical staining for extravasated plasma albumin on paraffin-embedded sections. Rats euthanatized after four to eight doses had no lesions, and albumin extravasation in the susceptible regions of the brain was minimal. Rats receiving 10 daily doses of TNB had bilaterally symmetric vacuolation and microcavitation in the cerebellar nuclei, vestibular nuclei, and inferior colliculi in association with multifocal, often confluent foci of extravasated albumin in susceptible nuclei. Albumin was present in vascular walls, extracellular space, and neurons. Immunoreactivity in neurons was of two types: cytoplasmic staining representing pinocytic uptake and homogeneous staining of the entire neuron (nucleus and cytoplasm) due to uncontrolled albumin leakage through the damaged cell membrane. In rats allowed to recover for 10 days, the microcavitated foci were infiltrated by glial and gitter cells. Albumin immunoreactivity was present as extracellular granular debris, and neuronal staining (for albumin) was mild. In rats allowed to recover for 30 days, immunoreactivity to albumin was not seen. This study demonstrates that TNB-mediated tissue damage is accompanied by breakdown of the BBB. The presence of vacuolation and associated extravasated serum proteins in TNB-treated rats is an indication of vasogenic brain edema, which appears to be a critical event in TNB toxicity. Additional studies are needed to determine the reason for selective regional vulnerability and brain microvascular susceptibility to TNB.