The mice infected with Japanese B Encephalitis were determined to the nucleic acid contents of their brain in connexion with the effects of P(32)-administration. The brains were fractionated into two fractions, DNA- and RNA-fractions by Schmidt-Thannhauser's method. On these two fractions phosphorus contents were determined by Fiske-Subbarow's method. In this experiment the virus was inoculated intracerebrally. The results are summarized as follows: 1. The brains were picked out at intervals after the inoculation of virus and phosphorus contents of their DNA- and RNA-fractions were determined. At the latent period, no definite differences were evidenced in the DNA-P- as well as in the RNA-P-contents as compared with those obtained from the normal mouse. However, a slight increase in the RNA-P-content at the endstadium of the latent period and also increase in the DNA-P-contents at the manifestation period were observed. 2. When 10 μc. of P(32) was administered to each one of the infected mice, the DNA-P-content of the brain at the manifestaion period showed a slight decrease, as compared with that obtained from the brains of infected mice with no P(32)-administration. 3. Administration of the same doses of P(32) to normal mice induced no appreciable change in the RNA-P-contents as compared with those of the normal ones. This was also the case in their DNA-P-contents determined 6 as well as 24 hours after the administration of P(32). From all these observations it is assumed that the β-ray irradiated from P(32) is as effective as to inhibit the virus multiplication in the brain to some extent.