The stereochemistry of hydride reduction of erythrinan-7, 8-diones was markedly affected by the bulkiness of the reagent and the polarity of the solvent : sodium borohydride in ethanol-tetrahydrofuran gave the 7β-hydroxy isomers and tetrabutylammounium borohydride in methanol gave the 7α-hydroxy isomers, stereoselectively. This can be explained in terms of fluctuation of the balance between product development control and approach control. The structures of the products were established by chemical and spectroscopic means. Their O-mesylates were hydrolyzed under basic conditions with epimerization to give a mixture of alcohols with a preference for the 7β-hydroxy derivatives. A similar epimerization of the mesyloxy group was also observed in lithium aluminum hydride reduction.