The ability of ethanol to affect hindlimb ataxia and body weight changes induced by methyl mercury was studied in rats. Animals treated with either water or ethanol increased in body weight during the experiment and showed no impairment of hindlimb movement. Rats treated with methyl mercury also increased in body weight but developed moderate hindlimb ataxia. Animals treated with ethanol and methyl mercury initially gained but subsequently lost weight and exhibited severe hindlimb ataxia. The results provide evidence that ethanol can potentiate methyl mercury toxicity in rats and, by implication, in humans.