Reduction of the nitroxide free radical (TEMPOL) and the pO 2 in a mouse liver was measured using an ESR spectroscopy. TEMPOL (280mM,5ml/kg) and India ink (2ml/kg) as an oxymetry probe were administered to an anesthetized mouse through the tail vein, and the ESR signal was monitored on the in-vivo or in-vitro liver and on the in-vitro blood. Results: (1) In a control mouse, the pO 2 of the liver was about 14 Torr, and ESR signal of TEMPOL decreased in height and disappeared, rapidly. (2) In a CCl 4 treated mouse and a portal veinligated mouse, the pO 2 was about 8 Torr and 3 Torr, respectively; the decay of TEMPOL was markedly delayed in comparison with a control mouse. (3) In a sacrificed mouse by cervical dislocation, the pO 2 was below 1 Torr; ESR signal of TEMPOL decreased slowly and did not disappear completely. (4) The gradual reduction of TEMPOL was observed in a mouse specimen which was obtained from the liver of a TEMPOL-treated mouse. (6) The reduction of TEMPOL did not occur in the in-vitro blood. Conclusions: (1) The nitroxide radical can be deoxidized in the liver, but not in the blood. (2) The reduction of TEMPOL was delayed in proportion to the decrease of pO 2 in the liver. (3) In a CCl 4 -treated mouse, the decreases of the pO 2 and reductants in the liver might involve the delay of the radical reduction.