The author investigated the influence of varying the components of Pfeffer solution on the production of flavinmononucleotide (FMN) and flavinadeninedinucleotide (FAD) by a strain of Aspergillus oryzae. FAD in mycelium greatly decreased and FMN gradually decreased with advancing days. The addition of riboflavin to the media seemed to cause no difference in the amounts of FMN and FAD. The presence of Fe‥, seemed to decrease the amounts of the both substances with the increasing concentrations. In case of Fe…, the optimum concentration was found to be ca. 10^<-5> moles per litre. The carbon source whatever kind it may be, exerted little influence, but 1 per cent of alcohol produced relatively larger amounts of FMN and o-ribose always gave low valus. As a nitrogen source, NO_3 was much superior to NH_4. No goood results were obained with organic nitrogen sources.