Somewhat detailed knowledge was obtained on the nucleic acid metabolism in long, spleen and kidney of the infected mouse P(32) was used as the tracer and was injected to each mouse intraperitoneally various hours after the intracerebral inoculation of the virus. 6 and 24 hours after the injection of P(32) the organs were collected and fractionated by Schmidt-Thanhauser's method into RNA-phosphoprotein fraction and DNA fraction, which were then examined on their radioactivities. Just at the incubation stage or the visceral phase of Japanese B encephalitis the organs, lung, spleen and kidney, of the mouse proved to have high radioactivities, both in their RNA-phosphoprotein fractions and DNA fractions. It is interesting to note that the radioactivity in DNA fraction was observed to increase rather late in the incubation stage while the other appeared from the first of that stage on. The fact that in case of spleen higher radiotivity of RNA-phosphoprotein fraction than that of DNA fraction was observed should further be pointed out. In contrast to this, the high radioactivities in acidsoluble fractions also observed at the mcnbation stage remained unaltered even at the acme stage of the disease.