Protein kinase CK2 has been found in two nuclear fractions obtained after treatment of purified rat liver nuclei with nucleases (S1 fraction) and subsequently with 1.6 M NaCl (S2 fraction). In both fractions three isoforms of the alpha subunit were identified. Two of them corresponded to the classical alpha and alpha' subunits, whereas the identity of the third one (alpha 3) remains unknown. In the S1 fraction two peaks of CK2 activity were detected at 6 h (5.5 fold) and 24 h (1.9 fold) after partial hepatectomy, whereas no significant changes were found in the S2 fraction. At 6 h after laparatomy a much lower increase of CK2 in S1 fraction was also detected (2.5 fold). The increases in CK2 activity found at 6 h after hepatectomy or laparatomy were accompanied with rises in the amount of the alpha subunit.