1. Acute riboflavin deficiency was produced in weanling rats by feeding a deficient diet and using tailcups to prevent refection. Animals were killed at weekly interval for 7 weeks, by the end of which they had become severely deficient, and mortality was high.
2. Growth of the deficient animals virtually ceased in the early stages of deficiency; food intake was severely and progressively depressed. Liver: body-weight increased markedly; packed cell volume fell at a late stage only. Pathological signs accumulated throughout the deficiency but were not closely related to the biochemical changes within the deficient group.
3. The activation coefficient (stimulated: basal activity; AC) of glutathione oxidoreductase (EC 188.8.131.52; glutathione reductase; GR) in erythrocytes rose to a mean value of 3–8 after 3 weeks, and subsequently remained almost constant: this change was not seen in pair-fed or ud lib.-fed controls. Both deficient and pair-fed animals exhibited a twofold reduction in FAD-stimulated erythrocyteGR activity at an early stage. In liver, both deficient and pair-fed groups showed a major progressive fall in FAD-stimulated GR activity, but only the deficient group showed an increase in AC, which occurred towards the later stages of the experiment. In skin, too, the deficient group showed an increase in AC during the terminal stages.
4. Hepatic, intestinal and brain succinate: (acceptor) oxidoreductase (EC 1.3.99,1; succinate dehydrogenase) activity fell relatively early during deficiency; in liver and intestine this was at least partly shared by the pair-fed group, and therefore attributable to inanition. Changes in hepatic NADH:(acceptor) oxidoreductase (EC I.6.99.3; NADH dehydrogenase) activity appeared to be entidy attributable to inanition.
5. An early reduction was observed in hepatic ATP: riboflavin 5-phosphotransferase (EC 184.108.40.206; flavo- kinase) activity in the deficient group, values falling by nearly half within 1 week, and then remaining almost constant. Similar but smaller changes were seen in renal flavokinase activity. Hepatic ATP: FMN adenylyltrans- ferase (EC 220.127.116.11; FAD pyrophosphorylase) was unchanged until the third week, at which point it rose sharply to a new plateau in the deficient group; in kidney it did not respond. These changes were not observed in pair-fed or ad-lib.-fed controls.
6. Hepatic flavin levels fell dramatically during the first 2 weeks of deficiency, FAD being conserved at the expense of FMN. Smaller changes were observed in kidney.
7. Of the processes which are affected by riboflavin deficiency, AC of erythrocyte GR (EGRAC) responds earlier, more dramatically and more specifically than most others, with the possible exception of hepatic flavin levels and flavokinase. Potentially, it is therefore a good index of over-all body riboflavin status, but in acute deficiency the rate of response of many variables is not related to the final extent of response; consequently the correlation between EGRAC and other riboflavin-sensitive processes is less satisfactory than it would be in an equilibrium situation.