Reactions of 3-isopropylidene- and 3-cyclohexylidene-2-ethoxyindolenine tetrafluoroborates with guanidine afford dihydropyrimido[4,5- b]indole derivatives containing a hydroxy group, unlike analogous reactions of 3-arylmethylidene and 3-(dimethylamino)methylidene tetrafluoroborates.