Acute changes in nucleic acid synthesis of murine hepatocytes after application of some kinds of agents were studied cytopathologically. The quantity of DNA in almost all normal liver cells shows accurate euploidy, such as 2c, 4c, 8c, etc., by microspectrophotometry (UMSPI, Carl Zeiss). This result corresponds with that of karyometry. Intranuclear quantity of RNA of the mouse and the rat is 15% of DNA. By using such normal in vivo liver cell, the following results were obtained. (A) Toxins or agents which injure DNA transiently reduce the extinction of DNA at a wave length of 260 nm. Microscopically the heterochromatin of the nucleus decreases and becomes thin. On the contrary, the nucleus of the cells in synthesisphase shows that the chromocentrums are reduced, the heterochromatin forms rather a fine network with the euchromatin, and the nucleoli become vague or unrecognizable. (B) On RNA-syn-thesis: (a) The liver cells in which RNA synthesis is decreased: In the case of the cells in which DNA-dependent RNA synthesis is relatively purely disturbed, intranuclear RNA decreases. Microscopically, the nucleoli become smaller and/or disappear and eosinophilic substance appears secondarily in the karyoplasm. In the case of the cells in which the ergastoplasm of the cytoplasm is disturbed first and migration of RNA from the karyplasm into the cytoplasm is stopped, intranuclear RNA increases markedly. Microscopically, the so-called giant nucleolus appears, but its stainability reduces. Distribution of the basophilic substance of the cytoplasm becomes markedly irregular. In the case of the cells in which both DNA and the cytoplasm are injured, intranuclear RNA increases. Microscopically most of the nucleoli become enlarged and their stainability is reduced. The nucleolus-associated chromatin decreases, especially markedly. (b) The liver cells, in which RNA synthesis is accelerated: Intranuclear RNA is normal or decreases. Microscopically the nucleoli become smaller and their stainability increases, that is, the nucleoli are concentrated. The nucleolus-associated chromatin becomes wider. The basophilic substance of the cytoplasm is distributed diffusely and “schollig”. (c) At the early stage after hepatectomy, intranuclear RNA increases and microscopically the nucleoli become larger, but more edematous and RNA synthesis are reduced, then intranuclear RNA decreases gradually. Microscopically, the nucleoli become smaller and the basophilic substance of the cytoplasm increases when RNA synthesis is just accelerated. Enlargement of the nucleolus at the latter stage is thought to be the true “reparative swelling”.