To explore the metabolic aspects of chronic liver disease, diurnal changes of arterial ketone body ratio (acetoacetate/3-hydroxybutyrate), reflecting hepatic mitochondrial redox potential, were investigated in normal subjects, patients with chronic liver disease (Child's class A or B), and patients with hepatic failure (Child's class C). Ketone body ratio in normal subjects increased after breakfast from 0.96 to 2.00, after lunch from 2.17 to 2.38, and after dinner from 1.23 to 2.55 with blood glucose level ranging from 103 to 141 mg/dL (5.7 to 7.8 mmol/L). By contrast, the ketone body ratio in the Child A or B group changed little and remained within a range of 0.70-1.35 despite a large change in blood glucose level from 102 to 176 mg/dL (5.7 to 9.8 mmol/L). Ketone body ratio in Child's class C remained near or below 0.4 with no response to glucose administration, despite a marked elevation in blood glucose level. These results indicate that hepatic mitochondrial redox potential undergoes diurnal changes in sharp response to meals in normal liver function but that these fluctuations are absent in patients with liver damage (Child's class A, B, and C). Furthermore, it remains at low levels in severe liver failure (Child's class C). It is also suggested that hepatic mitochondrial redox potential plays an important role in the regulation of blood glucose levels.