The authors report on a convenient electron impact ionization-mass fragmentographic method for determining the amount of tetrodotoxin (TTX) present in body fluids. To explain this method briefly, the TTX present in body fluids was purified with activated charcoal and transformed into 2-amino-6-hydroxymethyl-8-quinazoline (C9-base) with alcoholic KOH (1%). Then the C9-base was injected into a gas chromatograph-mass spectrometer in the form of trimethylsilyl derivative. In this manner the minimum detectable amount of TTX in body fluids was found to be about 0.01 microgram/g. When this method was applied to sera, urine, and the stomach contents of 2 decedents suspected of puffer fish poisoning, 0.053 microgram/g of TTX was detected in stomach contents of one victim. This new analytic method is simpler and more sensitive than other assay methods and thus is considered useful in forensic work.