The effect of methanol on the activity of hepatic δ-aminolevulinic acid synthetase (ALA synthetase) was investigated in male mice. The activity of ALA synthetase increased with increasing doses of methanol up to 100 mmol per kg of body weight injected intraperitoneally. In both the methanol-treated mice and normal mice, the apparent biological half-life time of the enzyme was about 56 min after the administration of cycloheximide. The increase of the enzyme activity by methanol was significantly inhibited by pretreatment with actinomycin D, an inhibitor of protein synthesis. The apparent Km value of the enzyme for glycine did not change but the V_max value increased in methanol-treated mice. Pretreatment of mice with pyrazole, a potent inhibitor of alcohol dehydrogenase, significantly inhibited the methanol-induced increase of ALA synthetase, whereas pretreatment with either 3-amino-1, 2, 4-triazole or acetanilide, inhibitors of catalase, did not alter the increase of the enzyme activity by methanol. It was concluded that methanol injected to fasted mice induces hepatic ALA synthetase and that the enzyme induction is associated with the oxidation of methanol by alcohol dehydrogenase.