ADP-ribosylation is a posttranslational protein modification catalyzed by two classes of enzymes: mono-ADP-ribosyltransferase and poly-ADP-ribose polymerases. We previously demonstrated that long-term alcohol intake remarkably enhanced an endogenous ADP-ribosylation of a 58 kDa protein in rat liver and also identified the 58 kDa protein as phosphoglucomutase (PGM). To assess biological significance of this phenomenon, we tested the effects of long-term alcohol intake on PGM activities in connection with posttranslational modification of the protein. ADP-ribosylation of PGM was mono- rather than poly-ADP-ribosylation. Also, nonenzymatic binding of ADP-ribose was excluded. It was of note that ADP-ribosylation of exogenous PGM was remarkably increased by adding rat liver plasma membranes, and that the extent of the increase was greater in alcohol-fed rats than in pair-fed controls. Furthermore, PGM activities were significantly increased after long-term alcohol intake concomitant with increased ADP-ribosyltransferase activities toward PGM. In view of the variety of roles of PGM in the liver, such as carbohydrate metabolism and Ca2+ homeostasis, it is tempting to speculate that increased ADP-ribosylation of PGM may play a role in long-term alcohol effects on hepatocytes.