Thiothiamine might be oxidized to thiamine in animal bodies coupled with any other biological oxidation. Therefore preventive tests of thiothiamine against thiamine deficiency on the simultaneous administration of cysteine, sodium thioglycollate, FAD, ascorbic acid or menadione were made with young rats. Ten μg of thiothiamine per day with any of them did not prevent thiamine deficiency : all animals lost weights after 2 or 3 weeks. Daily dose of 50μg thiothiamine alone showed no further growth, although a few animals were able to maintain their slow growth. Simultaneous administration of each of the compounds with thiothiamine was proved to have no appreciable synergistic effect for weight-gain of the animals compared to the response curve of daily 5μg doses of thiamine.