In golden hamsters, a study was made on the vascular permeability changes which might take place during the formation of triethyltin (TET)-induced brain edema. For this purpose, the animals received a single intravenous (i.v.) injection of TET sulphate (5-10 mg/kg b.wt) and groups of animals were studied 4 to 24 h thereafter. By the use of a new density gradient technique based on polyvinylcoated silica particles (1), it was shown that white matter edema was present already at 4 h after the TET injection. The edema then progressed during the following 20 h. Electron microscopy revealed that fluid accumulated in myelin vacuoles of the hamsters in the same way as has been described in other animal species. The macromolecular tracer, horseradish peroxidase mol.wt 40,000 injected i.v., did not leak out of the cerebral vessels during the period when edema developed. In order to find out if the formation of edema is associated with a vascular permeability increase to other and smaller markers, we used several fractions of FITC-dextrans varying from mol.wt 3,000 to 70,000 and determined their intracerebral localization with a histotechnical procedure. FITC-dextrans, mol.wt 70,000, did not leak out of the cerebral vessels in any of the TET intoxicated hamsters during the observation period of 24 h. The same was true for most animals given the other dextran fractions. However, FITC-dextrans, mol.wt 3,000-20,000 were present outside the vessels in the edematous optic nerves and corpus callosum in a few TET treated animals taken 16-24 h after the TET injection.(ABSTRACT TRUNCATED AT 250 WORDS)