Seven rhesus monkeys were subjected to one or more episodes of acute thiamin depletion. It is clear that significant metabolic inadequacies preceded demonstrable structural changes. Diminished food consumption and weight loss were manifest about two weeks after thiamin was removed from the diet. When the deficiency was prolonged the animals became apathetic, inactive and progressively weaker. This was followed by ataxia and at times ptosis and tremors. Even in such advanced states of depletion, administration of thiamin produced dramatic improvement in locomotion, appetite and reactivity. The blood thiamin content of normal monkeys ranged from 5.5 to 10γ per 100 ml. of whole blood, values which are comparable to those of healthy human beings. Following withdrawal of thiamin the blood concentration fell to values of 4γ or less. The tissue content of thiamin was correspondingly reduced in depleted animals. The minimum daily requirement for thiamin calculated on the basis of the time required to redeplete a deficient monkey following a small dose of thiamin was approximately 15γ per kilogram body weight. Characteristic degenerative changes in the heart muscle and severe retrogressive changes in the nuclear structures of the central nervous system previously reported were noted. Based on careful hematologic studies in 4 animals it is concluded that thiamin is essential for normal erythropoiesis. Acute or chronic depletion results in anemia due to suppression of red blood cell formation as indicated by severe depression or absence of reticulocytes in the blood.

ACKNOWLEDGMENT We are indebted to Miss Mariette Quigley for the blood counts, and to Miss Ruth Johnson for assistance in thiamin assays.