1. The measurement of glutathione reductase in white cells from peripheral rat blood is described; the contribution from red cell glutathione reductase was virtually eliminated. 2. The activation of the white cell enzyme by flavin adenine dinucleotide (FAD) in vitro was measured during an eight week period of dietary riboflavin deficiency in weanling rats. 3. The activation increased progressively during the deficiency reaching about fifty per cent after eight weeks, at which time liver FAD levels had declined to 30% of the control levels and clinical symptoms of deficiency were beginning to appear. No increase in activation of white cell glutathione reductase was observed in pair-fed controls, over this time period. 4. These observations suggest that the measurement of the activation coefficient of white cell glutathione reductase could usefully supplement that of the red cell enzyme, in defining riboflavin status.