The object of the present experiment is to determine the effect of thiamine administration on lactic acid concentration in blood plasma and the mineral metabolism in sheep given low and high concentrate rations. A high concentrate ration containing 90% concentrate and 10% roughage was fed to one group of sheep and a low concentrate ration containing 60% concentrate was given to the other group. Thiamine was intramuscularly injected into every sheep at a level of 50 mg of thiamine tetrahydrofurfuryl disulfide per day. No great difference was found in the plasma lactic acid concentration between the animals fed the low and high concentrate rations, and those between the control and thiamine injection periods. Thiamine injection appeared to increase urine and fecal magnesium excretion and significantly decreased magnesium balance (p less than 0.05). It may be conceivable that endogenous magnesium excretion is enhanced by thiamine administration in sheep.