By using alpha-phenyl N-tert-butyl nitrone (PBN) as spin trap molecule and the electron paramagnetic resonance (EPR) technique, we obtained the first direct evidence of in vivo intervention of free radicals during an ischemia (50 minutes) reperfusion phenomenon in kidney of an intact rabbit. An EPR signal (triplet of doublets) characterized by coupling constants aN = 14.75-15 G and aH beta = 2.5-3 G was detected in blood samples. The signal was consistent with a nitroxyl-radical adduct resulting from the spin trapping by PBN of either oxygen-or carbon-centered radicals. Control experiments indicated that the EPR signal was not due to a toxic effect of the spin trap molecule.