The definitive structure and the relative configuration of the title compound, perhydro-2(R)-[2(S)-hydroxypentyl]-6(S)-[2(R)-hydroxypentyl]-N-methylpyridinium chloride dihydrate, C16H34NO2+-Cl-.2H 2O, a new alkaloid isolated from Siphocampylus verticillatus that exhibits a potent antinociceptive activity, has been determined.