Brain ischemia was produced by occluding bilateral common carotid arteries of Mongolia gerbil with clips for 30-min and reperfusion was established by removing the clips. Superoxide dismutase (SOD) in brain tissue was measured by chemoluminescence, malondialdehyde (MDA) by fluorescence spectrometry and leukotrienes (LT) by bioassay. Decrease of SOD and increase of MDA were significant in the brain after 30-min ischemia followed by 2-min reperfusion; The level of SOD increased from 13.4 +/- 2.7 to 18.8 +/- 3.0, 19.8 +/- 2.5, 22.1 +/- 3.9 x 10(3) units/g brain tissue and MDA decreased from 218 +/- 26 to 169 +/- 41, 167 +/- 36, 167 +/- 44 nmol/g brain tissue respectively by ip silybin 100, 200 and 400 mg/kg 30-min before occlusion. LT decreased from 3.1 +/- 1.0 to 1.5 +/- 0.4 ng/g brain tissue after ip silybin 200 mg/kg. It suggested that the protective effect of silybin on ischemic brain result from the inhibition of 5-lipoxygenase and lipoperoxide and scavenging of oxygen free radical.