1. Reduced intake of energy or protein per se decreased riboflavin requirement of the chicks under restricted feeding regimens.
2. In severe riboflavin deficiency (2·26 mg/kg diet), efficiency of utilization of both energy and protein was decreased significantly under ad lib. conditions of feeding.
3. Efficiency of utilization of energy (but not protein) was decreased significantly in border-line riboflavin deficiency (3·02 mg/kg diet), when chicks were fed ad lib.
4. Efficiency of protein utilization was decreased significantly in severe riboflavin deficiency when chicks received an energy-restricted diet supplying 80, 70 or 60 yo of the energy intake of the controls.
5. Neither energy nor protein utilization was affected by riboflavin deficiency when both energy and protein were restricted to 70 yo of that consumed by chicks fed ad lib. However, the riboflavin deficiency produced characteristic deficiency symptoms, namely heterophilia, curled-toe paralysis, and reduction in concentration of liver riboflavin.