The mushroom hydrazine agaritine was measured in mouse plasma and urine using LC/MS/MS, which is highly specific. Agaritine concentration peaked 20 min after oral administration to mice (4.0 and 40 mg/kg). The concentration gradually decreased and returned to the basal level in 100 min. The maximum concentration, the time to the maximum concentration, and the half life were 0.37 microg/ml plasma, 0.33 h, and 0.71 h, respectively after administration of agaritine at 40 mg/kg body weight. One agaritine metabolite was found in the plasma and the urine from agaritine-administered mice. The structure of metabolites of agaritine by gamma-GT was next investigated using LC/MS. HMPH proved to be generated from agaritine. The oxidative stress marker 8-OHdG was detected in agaritine-administered mouse urine. After administration, the 8-OHdG level immediately tripled, and then decreased to the control level over 48 h. Its level then elevated again and remained high for 11 days. These results suggest that agaritine quickly metabolizes and disappears in the plasma, whereas DNA damage lasts for a long time after a single administration of agaritine to mice.