The pseudobenzylisoquinolines are a small group of alkaloids incorporating a benzylisoquinoline nucleus. Ring A is usually oxygenated at C-6 and C-7, while ring C is inevitably oxygenated at C-2',3', and 4'. It has been suggested that pseudobenzylisoquinolines could originate in vivo from the oxidative cleavage of protoberberinium salts, thus accounting for the unusual oxygenation pattern observed for ring C. Known pseudobenzylisoquinolines include polycarpine, polyberbine, taxilamine, (-)-ledecorine, and rugosinone. The occurrence of pseudobenzylisoquinolines must be fairly widespread because they have been reported in members of the Annonaceae, Berberidaceae, Fumariaceae, and Ranunculaceae. We have now observed the presence of the amorphous, new, yellow pseudobenzylisoquinoline alkaloid, dihydrorugosinone (1), in Chilean Berberis darwinii Hook. as well as in Berberis actinacantha Mart. ex Schult. (Berberidaceae). The 200 MHz CDCl3 1H nmr spectrum of dihydrorugosinone is essentially identical with that of the known synthetic dihydrorugosinone prepared by in vitro oxidation of berberine. Similarly, the uv spectrum of the natural product is close to that reported for the synthetic material. Significantly, B. darwinii also produces rugosinone. Because berberine is found in all Berberis species, it is quite possible that this quaternary alkaloid may be oxidized in vivo to dihydrorugosinone, which can then be dehydrogenated to rugosinone. The oxidation of dihydrorugosinone (1) to rugosinone has also been carried out in vitro using air and ethanolic NaOH.