Poly(ADP-ribose) synthetase catalyzes initiation, elongation, and branching of poly(ADP-ribose) on various acceptor proteins including itself. The automodification took place at about 15 sites on the synthetase molecule of rat liver; at each site was bound poly(ADP-ribose) as large as greater than 80 ADP-ribose units long with branched structures. Such extensive automodification accompanied a marked increase in the apparent Mr (to greater than 500,000) of the synthetase. Based on this unique property, the synthetase was identified as the main acceptor of poly(ADP-ribose) in isolated nuclei and permeabilized cells. The enzyme-bound poly(ADP-ribose) was not transferred to histone under the various conditions tested. These results suggested that the automodified enzyme does not serve as an intermediate in the modification of other proteins, but plays some biological role(s) as a structural element. ADP-ribosyl histone splitting enzyme, an enzyme responsible for the cleavage of ADP-ribose-protein bonds, was purified 5,000-fold to apparent homogeneity from rat liver. The purified enzyme was composed of a single polypeptide of Mr approximately equal to 80,000. The substrate specificity was broad for the protein portion, but strict for the ADP-ribose portion; only mono(ADP-ribosyl) proteins served as the substrates. The splitting reaction appeared to proceed by a non-hydrolytic mechanism producing a derivative of ADP-ribose with an altered terminal ribose.