We investigated free radical reactions in lung of living mice using an in vivo electron spin resonance (ESR) spectrometer and nitroxyl radical as a probe. When an aqueous solution of nitroxyl probe was trans-tracheally administered into lung of living mice, a sharp triplet signal was observed at the chest of the mice. The signal showed a gradual decrease with time, obeying first-order kinetics. Signal decay rates of carbamoyl-PROXYL and carboxy-2,2,6,6-tetramethyl-piperidine-N-oxyl were faster than those of CAT-1 and carboxy-PROXYL. The mechanism of signal decay may be attributed to (i) reaction with reactive oxygen species such as ·OH, (ii) transfer into blood circulation, or (iii) reduction by compounds continuously supplied. However, little is known about the clearance mechanism of the nitroxyl probe in lung. To evaluate the disappearance of the ESR signal of the nitroxyl probe in lung, in vivo ESR spectra in chest of mice was recorded after trans-tracheal administration of an aqueous high concentrate solution of nitroxyl probe. A broad signal from the chest was observed immediately after administration due to Heisenberg spin exchange interaction. A sharp triplet signal was superimposed on the broad signal and the appearance of a triplet signal was followed by disappearance of the broad one. Peak-to-peak line width of the sharp signal was almost the same as that after intravenous administration. A distinct signal was detected in blood collected 10 min after trans-tracheal administration of nitroxyl probe. The observations indicate the transfer from lung to blood circulation and its contribution to clearance of probe in lung. Appearance of a sharp signal in blood after trans-tracheal administration was dependent on the kind of nitroxyl probe, showing a different transfer rate from lung to blood.