The possibility that acetaldehyde accumulation potentiates the acute toxicity of ethanol was studied by pretreating rats with cyanamide, an aldehyde dehydrogenase inhibitor. At 30 min after administration of ethanol (7 to 9 g/kg, po), the levels of acetaldehyde in femoral venous blood of cyanamide-treated rats were increased from 10 to 20 to 600 mumol/liter and at death the concentrations of acetaldehyde in heart blood and cerebrospinal fluid were still 7 to 9 and 4 to 9 times higher, respectively, than in rats given ethanol only. The cyanamide pretreatment (25 mg/kg) significantly increased the mortality of rats given 6.5 to 7.0 g/kg ethanol and decreased the LD50 of ethanol from 7.3 to 5.9 g/kg. Cyanamide increased the late mortality, possibly because of sustained acetaldehyde accumulation. Although administration of the alcohol dehydrogenase inhibitor, 4-methylpyrazole (4-MP, 10 mg/kg), prevented the accumulation of acetaldehyde, it only partly counteracted the effect of cyanamide on mortality. After coadministration of cyanamide and 4-MP, the LD50 of ethanol was 6.5 g/kg, and after 4-MP alone, 6.7 g/kg. 4-MP by itself seemed to increase the early mortality of rats to ethanol poisoning. The results suggest that the potentiating effect of cyanamide on ethanol toxicity can partly be explained by acetaldehyde accumulation and that 4-MP can be used to inhibit this accumulation providing its central depressant effect is taken into account.