The contents of insulin-like substances in sera of mice were determined by the isolated mice diaphragm method. In mice with thiamine deficiency or alloxan diabetes, those contents were less than those in the normal ones. The glucose uptake of isolated mice diaphragms, was also fond decreased in the former two. However, when thiamine was admistered to them, the glucose uptake became normal. When thiamine was added to the medium, the glucose uptake of the normal isolated diaphragm was increased considerably. But the glycogen synthesis from the taken glucose was not increased when thiamine was added in vitro, while in vivo thiamine had no influence at all on the insulinase activity.