The tissue distribution and the urinary excretion of hydrazines, hydrazine, acetylhydrazine and 1,2-diacetylhydrazine, were determined by mass fragmentography using a gas chromatography-mass spectrometer equipped with a multiple ion detector-peak matcher. Using the compounds labeled with a stable isotope as an internal standard, namely the isotope dilution method, made it possible to estimate trace amounts of hydrazine and its metabolites in the tissues. Significantly high levels of all hydrazines were detected in the kidney. Especially, acetylhydrazine, a metabolite of hydrazine, accumulated to a great extent in the kidney. Free hydrazine which was liberated from acetylhydrazine was detected both in the tissues and in the urine after the administration of acetylhydrazine. This demonstrates clearly that the metabolic pathway between hydrazine and acetylhydrazine is reversible.