Changes in hepatic levels of lactate, pyruvate, phosphoenolpyruvate, alpha-ketoglutarate, malate, oxaloacetate, adenine nucleotides, inorganic phosphate, ketone bodies, alanine, serine, glycine, aspartate, glutamate, valine and urea were examined in adult rats during the first 24 h of either starvation or consumption of a high protein (HP) diet. No differences were found between these two conditions in the concentration of metabolites studied or the cytosolic redox state. Under both conditions, the cytosolic phosphorylation state decreased to a low 15 h into the experiment but the changes were more pronounced on the HP diet. Hepatic ketone bodies rose sharply after 12 h, with the increase 2.5 times greater for starved rats. In starvation, hepatic aspartate, valine, and urea were low and glycine was high, whereas the opposite was seen for the HP diet. In both groups, alanine fell within 9 h and remained low thereafter. These findings suggest that, in the first 24 h of starvation, the energy necessary for gluconeogenesis is obtained from fatty acid oxidation, while during HP feeding the energy for both gluconeogenesis and ureagenesis are derived from fatty acid oxidation and amino acid oxidation.