1. We have investigated the permeability-increasing effect of arachidonic acid on pial venular capillaries in vivo using the single microvessel occlusion technique. 2. Permeability to Lucifer Yellow dye (476 Da) increased dose dependently when arachidonic acid was applied focally to the abluminal surface of the vessels. This was completely reversible at all but the highest dose. The permeability increase was 1.65 x 10(-6) +/- 0.247 x 10(-6) cm s-1 (mean +/- S.E.M.) at 0.25 mM, 3.53 x 10(-6) +/- 0.872 x 10(-6) cm s-1 at 0.5 mM, 12.61 x 10(-6) +/- 3.44 x 10(-6) cm s-1 at 1 mM and 18.46 x 10(-6) +/- 5.90 x 10(-6) cm s-1 at 2 mM arachidonic acid. There was a similar reversible dose-dependent permeability increase to eicosapentaenoic acid. 3. These permeability increases could be prevented by co-application of a mixture of the antioxidants superoxide dismutase and catalase (30 and 100 U ml-1), or by the iron chelator desferrioxamine (100 microM). 4. The permeability increases were also prevented by the cyclooxygenase and 5-lipoxygenase blockers indomethacin (100 microM) and nordihydroguariaretic acid (100 microM), respectively, when applied together, but not singly. 5. It was concluded that the permeability response to arachidonic acid depends on the formation of free radicals and subsequent lipid peroxidation.