To clarify the effects of partial ischemia and reflow on the mitochondrial metabolism of the rat liver, the afferent vessels supplying the left lateral and left half of medial lobes were occluded and then reperfused after given time periods of ischemia (30, 60, 90 and 120 min, groups A, B, C and D, respectively). Samplings were taken at 0, 10 and 60 min after reperfusion. The energy charge levels of ischemic lobes decreased rapidly from 0.85 ± 0.01 in the sham group to 0.38 ± 0.11, 0.35 ± 0.07 and 0.34 ± 0.06 in groups B, C and D, respectively. The phosphorylative activities of mitochondria isolated from ischemic lobes decreased gradually along with the time of ischemia. The reversal of mitochondrial function and energy charge levels following reperfusion was noted in groups A and B. In nonischemic lobes, the phosphorylation rate (nmol ATP/mg/min) increased from 90 ± 6 in sham group to 125 ± 12 and 130 ± 9; 131 ± 5 and 130 ± 6; 123 ± 6 and 122 ± 17, and 138 ± 6 and 138 ± 13 at 10 and 60 min after reflow in groups A, B, C and D, respectively (p < 0.05). The energy charge level of nonischemic lobes decreased from 0.85 ± 0.01 of sham group to 0.80 ± 0.03 in group D (p < 0.05). From these results, it is concluded that the transitional zone for the reversal of mitochondrial function and energy metabolism following prolonged liver ischemia appears at around 60 min. It is suggested that there exist some mechanism(s) which lead to an enhancement of mitochondrial function in nonischemic lob