A method for the quantitation of diesterditerpene-type Aconitum alkaloids and their hydrolysis products by gas chromatography-selected ion monitoring was applied to a clinical case study. A 45-year-old male attempted suicide by oral intake of Aconitum alkaloids, which are highly intoxicant extracts of Aconitum tubers. It was estimated that he had ingested approximately 11 mg of diesterditerpene-type alkaloids but was saved by intensive gastric irrigation. Mesaconitine, aconitine, hypaconitine, and their hydrolysis products were detected in the serum on the first day only. On the other hand, some alkaloids were still detectable in the urine even six days after intoxication. Aconitum alkaloids are biotransformed, and their hydrolysis products are excreted time-dependently to the urine. The urine was a useful material to identify the toxicants in the case of aconite intoxication.