We show that brief periods of fasting induce functional changes similar to those induced by long-term dietary restriction in mice, and these changes include protection from ischemia/reperfusion (I/R) injury. In this study, we investigated the mechanisms of protection induced by fasting, and we determined the effect on liver regeneration after partial hepatectomy. Partial hepatic ischemia (75 minutes) was induced in ad libitum fed mice and in 1- to 3-day-fasted mice, and one-third or two-thirds hepatectomy was performed in ad libitum fed mice and 3-day-fasted mice. Preoperative fasting for 2 or 3 days significantly decreased hepatocellular I/R injury. Hepatic gene expression of heme oxygenase 1 (HO-1), superoxide dismutase 2 (SOD2), glutathione peroxidase 1 (Gpx1), and glutathione reductase (GSR) was significantly up-regulated in 3-day-fasted mice at the baseline and 6 hours after reperfusion. After reperfusion, p-selectin and interleukin-6 (IL-6) levels were significantly lower, and superoxide radical generation, lipid peroxidation, and neutrophil influx were significantly attenuated in 3-day-fasted mice. Preoperative fasting did not affect liver regeneration after one-third hepatectomy. Hepatic gene expression of IL-6 and transforming growth factor β1 was significantly higher in 3-day-fasted mice before and after one-third hepatectomy. Tumor necrosis factor α expression significantly increased after one-third hepatectomy in 3-day-fasted mice. After a 3-day fast and two-thirds hepatectomy, liver regeneration and subsequent postoperative recovery were compromised. In conclusion, up-regulation of the stress response gene HO-1 and the antioxidant enzymes SOD2, Gpx1, and GSR at the baseline and a better response after reperfusion likely underlie the protection induced by fasting against hepatic I/R injury. Preoperative fasting may be a promising new strategy for protecting the liver against I/R injury during liver transplantation and minor liver resections, although its effect on extended hepatectomy warrants further exploration. Liver Transpl 17:695-704, 2011.