1. 1. We have fractionated and partially purified several rat liver nuclear protein kinases by utilizing endogenous (nonhistone proteins) and exogenous acidic (dephosphophosvitin) and basic (lysinerich histone) protein substrates. 2. 2. Three enzymes were active towards endogenous substrates, two towards dephosphophosvitin and two towards lysine-rich histone. 3. 3. Of the latter only one was cAMP-dependent. however, only minimal cAMP binding activity was detected. 4. 4. Several features of these enzyme reactions are described revealing distinct differences in the characteristics of each of these enzymes.