The effects of diets restricted in thiamine and riboflavin were studied for 3 yrs. in 36 [male] patients whose ages ranged from (1) 58-78 yrs., and (2) 24 to 42 yrs. Each group was divided to subgroups A, B, and C. Each person in the A groups received a daily diet of about 2,000 calories, deficient in thiamine (400 [gamma] present) and riboflavin (900 [gamma]). The B groups received the same diet plus yeast extract which contained about 6 mg. of thiamine and 1.3 mg. of riboflavin. The C groups received a regular hospital diet. The vit. restriction in the A groups produced only minimal effects. Activity and ambition were depressed. There was aging of the skin. An unusual elevation of blood lactic and pyruvic acids occurred. At the start of the 3d year the B groups were rigidly restricted in thiamin (200 [gamma]) and riboflavin (800 [gamma]). This produced elevation of blood lactic and pyruvic acids, swelling of the facial skin, budding of blood vessels into the cornea, paranoid mental changes, decrease of vibration sense in the lower extremities, and exaggeration of psychotic complaints. The older patients were affected earlier and more severely. Recovery was dramatic when yeast extract providing 6 mg. of thiamine was added to the diet.