Liver regeneration after partial hepatectomy was studied in rats 3 months after a single inj. of lasiocarpine N-oxide. In the 72 hours following hepatectomy the rate of regeneration in the alkaloid-treated rats was not less than in the controls. DNA synthesis by parenchymal cell nuclei also appeared to be unaffected, but the capacity of these nuclei for mitotic division was greatly reduced. Consequently, regeneration was accomplished by an increase in the size of parenchymal cells and the ploidy of their nuclei, and not by an increase in their number. In the alkaloid-treated rats the few mitotic figures observed were mostly abnormal. (AS).