Measurements by direct low frequency EPR spectroscopy provide the first evidence that Cr(V) is generated in one-electron reduction of Cr(VI) in live mice. The Cr(V) yield reached a maximum about 10 min following Cr(VI) intravenous injection and then decayed slowly with a life time of approximately 37 min. The time for the Cr(V) EPR signal to reach maximum intensity increased with the dose of Cr(VI). A discernible EPR signal was still observable at a dose as low as 0.1 mmol/kg. The Cr(V) was found predominantly in the liver, with a small amount in the blood. No Cr(V) signal was detectable in heart, spleen, kidney, and lung. Pretreatment of the animals with metal ion chelators, ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, and 1,10-phenanthroline significantly reduced the intensity of the Cr(V) signal that was observed.