Thermopsine, an isomer of anagyrine, is associated with the latter in nature, but differs from it in chemical reactivity. Evidence, both chemical and from the infrared spectra, indicates that the molecule of thermopsine contains an α-pyridone ring, and possesses a structure closely related to that of anagyrine. Catalytic reduction of the alkaloid yields α-isosparteine, a stereoisomer of sparteine. It is therefore suggested that thermopsine is a stereoisomer of anagyrine, to which it bears the same relationship as α-isosparteine to sparteine. The stereochemistry of these bases is discussed.