Tetrodotoxin (TTX, 1), a potent neurotoxin first isolated from puffers and then from California newts, has recently been reported from various biota. Its use as a sodium channel blocker is also expanding. Yet we know little of its biosynthesis or natural analogues. Major obstacles are the lack of a detection method for TTX analogues and the poor resolution of ¹H and ¹³C NMR spectra of TTX due to the hemilactal-lactone tautomerism. We previously isolated 4-epiTTX (2) and 4,9-anhydroTTX (3) from puffers, both of which are derivable from TTX with acids and thus provide little information about metabolic pathways. Significantly, we have now isolated from the newt Cynops ensicauda 6-epiTTX (4), 11-deoxyTTX (6), and their conversion products (5, 7, 8). The newts were extracted with hot 0.1% HOAc, and the extracts were chromatographed successively on columns of charcoal, BioGel P-2, BioRex 70, and Hitachi 3011C ion exchange gel. Separation of TTX analogues was monitored by a TTX analyzer and TLC. The structural determination of the analogues was achieved mainly through NMR measurements, including ¹H-¹H and ¹³C-¹H COSY measurements, which allowed us to assign for the first time all ¹H and ¹³C signals of TTX by adding CF₃COOD to the solvent. 6-epiTTX (4) has the same molecular formula as TTX (C₁₁H₁₇N₃O₈), exists as hemilactal-lactone tautomers, and has an LD₅₀ of 60 µg/kg (ip) to mice. 11-deoxyTTX (6) has the molecular formula C₁₁H₁₇N₃O₇, crystallized as colorless needles, has an LD₅₀ of 71 µg/kg (ip) to mice, and exists as hemilactal-lactone tautomers. These new analogues provide information about the metabolic pathways of TTX and are important for structure-activity relationships.