[en] Radioactive nitrogen-13 from nitrite (NO₂^-) or nitrate (NO₃^-) administered intratracheally or intravenously without added carrier to mice or rabbits was distributed evenly throughout most organs and tissues regardless of the entry route or the anion administered. Nitrogen-13 from both anions was distributed uniformly between plasma and blood cells. Rapid in vivo oxidation of NO₂^- to NO₃^- at concentrations of 2 to 3 nanomoles per liter in blood was found. No reduction of ¹³NO₃^- to ¹³NO₂^- was observed. A mechanistic hypothesis invoking oxidation of ¹³NO₂^- by a catalase-hydrogen peroxide complex accounts for the results