Administration of thiobenzamide in a single dose (25 mg/100 g body wt by stomach tube) to male rats induced centrilobular necrosis, which became evident 10 h after the poisoning. In the meantime liver weight and water content underwent changes, glycogen was lost, triglycerides accumulated in the liver while decreasing in serum, [3H]leucine uptake in proteins was impaired and the activity of glucose-6-phosphatase and aminopyrine demethylase decreased. The activity of NADPH-cytochrome c reductase remained unchanged, whereas a reduction of the microsomal cytochrome P-450 occurred. The liver amount of reduced glutathione underwent no significant changes. Pretreatment of the animals with cobalt chloride or 20-methylcholanthrene decreased the liver damage caused by the drug. The in vitro addition of thiobenzamide to liver microsomes resulted in a spectral change. The appearance of conjugated dienes among microsomal lipids from drug-treated rats indicated for a lipoperoxidation taking place in vivo.