It was observed that the nitrate reductase of silk-worm was not extracted with water or phosphate buffer by grinding but decreased the activity, for example in case of living tissue the activity of the residue once extracted was about 28% of that of the original enzyme suspension and 8.8% in the twice-treated one. In acetone powder, though the grade of the diminution of the enzymic action was small, e. g. to about half by twice extraction, the effect of extraction on the nitrate reductase had the same tendency. It is very probable that the decrease of the enzymatic activity by extraction may be explained in part as due to the removal of some substances which provide hydrogen to nitrate by catalytic action of proper dehydrogenase coexisting in the enzyme preparation. I assumed that when a hydrogen donator was furnished, the enzymic action must have been activated to some extent and the dehydrogenase concerned would be decided. Therefore, the same quantity of substances as the NO3 to be reduced was added to reaction mixture and the NO2, produced was determined. The restoration of activity of tissue which was extracted two times was attained according to the order in the following table. (snip) and in case of sugars by acetone powder : However, from the results in these studies there is reason to assume that the reactivation would be attained by other compounds which were reaction products of the substances added by other enzymes because the reaction time was 22 hrs which was too long to estimate the effects of H-donators. I tried, therefore, to use the Thunberg's tube in which NO3and enzyme solution were mixed after careful evacuation so as to shorten the reaction time and allowed the reaction to proceed at 400 for 1 to 2 hrs. It was found that these condition were satisfactory. In this circumstance that the effect of extraction was also the same as that in long reaction period was shown. (snip) It may be said that malic-, maleic , succinic , malonic , fructose- , sucrose-, glutamic-, and asparagine-dehydrogenases were concered to the nitrate reduction in the silk-worm.