In part III of this series data were presented for the changes in air following periods of anaerobiosis in the rate of CO
2
production of potato tubers and in the contents of sugar, lactic acid and other constituents. Here these experimental data are analyzed and further discussed. The time curve for decrease in the content of lactic acid in air following a period of anaerobiosis appeared to be nearly linear initially with a sharp inflexion as the air value of lactic acid was approached. For a given content of lactic acid the rate of loss of the acid was the more rapid, the shorter the period of anaerobiosis. Preliminary data for the changes in the content of pyruvic and other keto-acids in air following nitrogen were mentioned and the forms of the curves for loss of lactic acid were considered in relation to the system pyruvic acid + Co I. H
2
⇌ L-lactic acid + Co I lactic dehydrogenase The possible influence of changes both in the content of pyruvic acid and in the quotient Co I. H
2
/Co I on the form of the lactic acid content/time curve was noted. It was provisionally suggested that the effective activity of lactic acid dehydrogenase might decrease progressively in nitrogen and that this loss of activity might not be quickly reversed in air following nitrogen; alternatively in air following nitrogen, owing to the accumulation of reduced compounds during anaerobiosis, the quotient Co i.H
2
/Co i might for a time be maintained larger the longer the previous period of anaerobiosis. The CO
2
production in the after-effect was shown to have a dual origin, being derived partly from lactic acid and partly from sugar. The view was advanced that lactic acid was first oxidized to pyruvic acid, which was then transformed, either in part or completely, into other acids, possibly via the Krebs cycle. The keto-acids of the Krebs cycle may thus be the immediate substrates of the CO
2
production which is derived from lactic acid. The quantitative evaluation of the share of the two components, i. e. the non-sugar and the sugar CO
2
components, in the total CO
2
production, and the elucidation of the fate of the lactic acid presented serious difficulties. The analysis of the CO
2
production/sucrose relation during the after-effect in dicated that when lactic acid had decreased to the low level characteristic of aerobic conditions the CO
2
production was, for a time which varied in extent in the different experiments, approximately proportional to the sucrose concentration; how ever, in comparison with the values for samples held through out in air, the proportionality factor, i. e. CO
2
production/sucrose, was depressed to a greater or lesser extent in different experiments. If it was assumed first that the depression of sugar respiration during the time when lactic acid was disappearing was no greater than after the acid had decreased to the air-level and second that the respiration of sugar continued normally in the after-effect unaffected by the simultaneous oxidation of lactic acid, only a part of the lactic acid loss could be accounted for by CO
2
production; it was suggested that the residue of the lactic acid was either in part metabolized to other compounds, e. g. other organic acids, or was in part resynthesized to carbohydrate as in frog’s muscle (Meyerhof 1930). If, however, the respiration of sugar was assumed to be partly suppressed by the increased concentration of pyruvic acid arising from the rapid oxidation of lactic acid, then a greater proportion but not the whole of the lactic acid loss could be accounted for as CO
2
production; in this case, in addition to conversion to other organic acids and possibly resynthesis to carbohydrate as already mentioned, a part of the lactic acid would be oxidized in stead of sugar and so spare the normal consumption of sugar in respiration. The results confirm the observations of Singh (1927) on CO
2
production in the after-effect and extend them by the information provided by the data for the concomitant changes in the contents of lactic acid and sugar.
