Rats were separated into four groups and four different liquid diets were given to each group. Group 1: thiamine-sufficient diet with no ethanol, group 2: thiamine-sufficient diet with ethanol, group 3: thiamine-deficient diet with no ethanol, group 4: thiamine-deficient diet with ethanol. After four weeks, all rats were fasted for 24 hr and then ethanol was given orally to every rat. After one hour, every rat was sacrificed and biochemical and histological analyses were carried out. Transketolase activity in brain and liver decreased in groups 2, 3 and 4. There was significant decrease in transketolase activity in ethanol-fed groups (groups 2 and 4) as compared to control groups (groups 1 and 3). Ethanol concentrations in blood, liver and heart of rats in groups 2 and 4 were higher than in groups 1 and 3. When comparison was made between the thiamine-deficient groups and the corresponding thiamine-sufficient groups, ethanol concentrations in liver and heart were higher in the thiamine-deficient groups. Alcohol dehydrogenase activity in liver decreased significantly in groups 2 and 4. By histological analyses, fatty degeneration was observed in the livers of groups 2 and 4. The degeneration was more prominent in group 4 than group 2. These findings suggest that chronic ethanol administration may impair the ability to metabolize ethanol and the impairment may increase when rats are in the condition of thiamine deficiency.