4'-Methoxypyridoxine (1), an antivitamin B6, was isolated from the seed of Ginkgo biloba L. (Ginkgoaceae). The albumen of G. biloba seed, termed "Gin-nan" or "Ginkyo", is used as a crude drug in China and food in Japan, but approximately 70 cases of "Gin-nan sitotoxism" (cardinal symptom: convulsions; lethality ~27% in infants) have been reported in Japan, with the cause previously unclear. To identify the causative agent, the albumen was dried at 40°C for 7 days, ground, and processed via reported methods. Acute oral toxicity tests in guinea pigs revealed test samples induced characteristic symptoms (leg paralysis, opisthotonus, tonic convulsions, auditory hyperalgesia) within 30-40 minutes. Compound (1) was isolated with a yield of ~0.01% dry weight and induced convulsions at an oral dose of 11 mg/kg. Structural identification: High-resolution mass spectrometry confirmed the molecular formula C9H13NO3 (M+ 183.0890, calculated 183.0891); 1H- and 13C-NMR indicated an aromatic methyl group, methoxy group, two -CH2O- groups, an aromatic proton, and a tetrasubstituted pyridine ring; positive reactions with ferric chloride and Fast Blue B Salt suggested a hydroxypyridine with an unsubstituted para position; coupling with Gibbs reagent in the presence of borate confirmed 4'-substitution; mixed melting point (179-180°C) with authentic 4'-methoxypyridoxine hydrochloride (mp 181°C) finalized the identity as 4'-methoxypyridoxine. Known for antivitamin B6 activities and synthetic convulsive effects, 4'-methoxypyridoxine is reported here for the first time from natural products and is the substance responsible for "Gin-nan sitotoxism". Pyridoxine (vitamin B6) may prevent or alleviate symptoms by countering vitamin B6 deficiency induced by compound (1), and vitamin B6 scarcity during food shortages may contribute to the sitotoxism.