In order to evaluate the liver mitochondrial response to an increased energy load, eight groups of rats were studied: a control group (no surgery), a sham-operated group, and 6 groups that had undergone partial hepatectomy (PH). The PH rats either had no infusion following surgery, saline, isocaloric lipid, hypercaloric lipid, isocaloric glucose or hypercaloric glucose infusion. After isolation of liver mitochondria 6, 12, 18 and 24 h after surgery states 3 and 4 of respiration were measured in vitro in the presence of succinate. State 3 and 4 mitochondrial respiration was higher in the sham-operated rats 6 h after surgery than in the control animals, and a gradual decrease towards control values occurred in the 12-24 h period following surgery. Results in mitochondria from PH rats showed variable results, in most cases both State 3 and 4 respiration was higher after PH, while the respiratory control index in general was lower. State 3 respiration was significantly increased in the PH isocaloric lipid and glucose infused groups 6, 12 and 18 h, but not 24 h after partial hepatectomy. The PH hypercaloric glucose and hypercaloric lipid infused rats showed decreases in the liver mitochondrial oxygen consumption after 6, 12 and 18 h of liver regeneration, this was more pronounced after hypercaloric lipid infusion. These results suggest that, in vitro, the surgery tends to increase Status 3 and Status 4 mitochondrial respiration. The hypercaloric parenteral nutrition, both lipid and glucose based, significantly decreased liver mitochondrial oxygen consumption in partially hepatectomized rats, particularly in the first hours of liver regeneration, with restoration 24 h after surgery.