A study of the alkaloids present in the roots of Aconitum septentrionale Koelle has led to the isolation of four new norditerpenoid alkaloids: 8-O-methyllycaconitine (1), 6-O-acetylacosepticine (2), acoseptrigine (3), and acoseptriginine (4), besides seven known alkaloids: lappaconine (5), N-acetylsepaconitine (6), puberaconitine (7), lappaconitine (8), N-deacetyllappaconitine (9), lycoctonine (10), and lapaconidine (11). The structures of these alkaloids were determined by spectral data and chemical correlation with alkaloids of established structures. Thus, 8-O-methyllycaconitine (1) was converted to septentrionine (12) by treatment with methanolic ammonia. Saponification of 6-O-acetylacosepticine (2) and acoseptrigine (3) with 5% ethanolic KOH yielded acoseptricine (13) and 14-O-methylforesticine (14), respectively. A synthetic sample of lappaconine (5) was prepared from lappaconitine (8) by alkaline hydrolysis. Acidic hydrolysis of N-acetylsepaconitine (6) afforded sepaconitine (15). N-acetylsepaconitine (6) and puberaconitine (7) have not been previously isolated from A. septentrionale. 13C chemical shifts for C(5), C(10), C(13) and NCH2CH3 of lapaconidine (11) have been revised on the basis of DEPT measurements. © 1992.