In a reinvestigation of the alkaloids of the title cactus species, two trace alkaloids were isolated: heliamine (6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline) (0.007% yield), previously known from other cereoid cacti, and 1,2-dehydrosalsolidine (1-methyl-6,7-dimethoxy-3,4-dihydroisoquinoline) (0.006% yield), a novel fluorescent, tertiary amine. Mikes (ms/ms) analysis of the plant material and extracts suggests that the new alkaloid is not an extraction artifact. This is the first isolation of a dihydroisoquinoline from the Cactaceae; such compounds are speculated to be precursors of the more common tetrahydroisoquinolines; the use of ms/ms to detect traces of potential biosynthetic intermediates is emphasized. © 1982, American Chemical Society. All rights reserved.