Since it was shown previously that about 50% of patients with thyrotoxicosis display a marked decrease in plasma prothrombin, the effect of vitamin K on these patients was studied. Vitamin K (menadion) was given parenterally and vitamin K1 (phytomenadion) orally. Administration of these vitamins produced a temporary rise in the previously decreased prothrombin level, followed after a few days by a return to the initial value. The decreased prothrombin level was due to disturbed liver function, probably caused by liver damage. Since oral administration was as effective as that given parenterally, absorption from the intestine appeared unimpaired.