Two protein kinases, designated NI and NII, have been isolated from rat liver nuclei. These enzymes have a similar pH optimum and phosphorylate phosvitin and casein more readily than histone. Both enzymes require magnesium for activity. In the absence of Mg2+, other divalent cations such as Ca2+, Co2+, and Mn2+ can substitute partially for Mg2+ when the reaction is catalyzed by NI. With NII, only Co2+ showed any activity in the absence of Mg2+. Magnesium decreased the apparent Km for ATP of protein kinase NI without changing the Vmax of the reaction, and decreased the apparent Km's for both ATP and casein, while increasing the Vmax of the reaction threefold with protein kinase NII. Both enzymes are stimulated about twofold by low concentrations (0.1–0.3 M) of NaCl, KCl, and sodium acetate, whereas higher concentrations (> 0.5 M) inhibit their activities. Both enzymes are inhibited by low concentrations of NaF (0.02 M) and (NH4)2SO4 (0.1 M). NI and NII were found to have sedimentation coefficients of 3.6 S and 10.8 S, respectively. The nuclear protein kinases are not activated by cyclic AMP or cyclic GMP, and are not inhibited by the heat-stable cyclic AMP-dependent protein kinase inhibitor.