Monoacetoacetin, the monoglyceride of acetoacetate, was investigated as a nutritional support for the regenerating liver. Following partial hepatectomy, rats were either fed an oral diet ad libitum or administered by total parenteral feeding glucose alone, monoacetoacetin-glucose mixture, or lipid emulsion-glucose for the nonprotein calories. Five rats from each treatment were killed at 6-hr intervals beginning 12 hr after partial hepatectomy and ending at 72 hr. The number of cells synthesizing DNA and the number of cells in mitosis were compared. Rats fed orally or infused with glucose alone or with lipid emulsion had similar parameters throughout. Rats infused with monoacetoacetin had approximately double the number of cells in mitosis and DNA synthesis compared to the other treatments. This stimulation by monoacetoacetin persisted 72 hr. It was concluded from the data that acetoacetate was the agent responsible for increased DNA synthesis and mitosis, but the mechanism for the stimulation was not identified.