The effects of metallothionein and cadmium ion on the hepatic aldehyde oxidase activity and brain lipid peroxidation were tested in vitro. The content of brain lipid peroxide at the condition of normal or $300{mu}M$ Fe(II)-induced was remarkably reduced by the addition of metallothionein in the incubation mixture. The induced content of lipid peroxide by cadmium $(30{mu}g/ml)$ ion was reduced by metallothionein $(100{mu}g/ml)$. The activity of aldehyde oxidase was not affected by metallothionein, but cadmium ion $(8.38{mu}g/ml)$ increased the activity of aldehyde oxidase about 80% compared to the control. The cadmium-induced activity of aldehyde oxidase was restored to the control level by metallothionein or penicillamine.