Two structurally unidentified alkaloids (tentatively named bases III and IV2)), isolated from Menispermum dauricum DC. (Menispermaceae), were found to be dauriporphine (1), a known oxoisoaporphine-type alkaloid, and 2,3-dihydromenisporphine (3), a new alkaloid of the same type, respectively. The structure of dauriporphine was confirmed by synthesis of 4,5,6,9-tetramethoxy-7H-dibenzo[de, h]quinolin-7-one (1). © 1985, The Pharmaceutical Society of Japan. All rights reserved.