The nuclear content of conjugated polyamines increased during rat liver regeneration. Conjugated polyamines isolated from the acid-precipitable fraction of nuclei required peptide bond hydrolysis for release of the parent compounds. The most striking change occurred in conjugated putrescine which fluctuated in a biphasic manner; maximal nuclear levels 12-fold and 25-fold above those of sham-operated controls were achieved at 4 and 42 hr after hepatectomy, respectively. Conjugated spermidine and spermine increased 3- and 2-fold respectively within 4 hr and remained high throughout the 48 hr studied. When expressed on the basis of mg of nuclear protein, the maximal conjugated putrescine increased 19-fold, conjugated spermidine increased 2-fold, and conjugated spermine decreased by 50%. Therefore, the spermidine and spermine conjugates may be of a more constitutive nature whereas the large changes in the nuclear conjugation of putrescine associated with the onset of growth may play a regulatory role. The nucleus also contained transglutaminase (R-glutaminyl-peptide:amine gamma-glutamyl-yltransferase, EC 18.104.22.168), an enzyme shown in vitro to conjugate polyamines covalently to proteins. The specific activity of the nuclear enzyme increased rapidly after partial hepatectomy to a level 3-fold above control at 4 hr and 7-fold above control at 42 hr. The increased conjugating activity resulted from an increase in detectable maximal velocity and not a change in affinity of the enzyme for putrescine (Km congruent to 0.4 mM). There was also a 3-fold increase at 42 hr in the number of nuclear amine acceptor sites present to which radiolabeled putrescine could be conjugated by endogenous enzyme.