The present study examines the effect of four new protopine alkaloids (1-4) isolated and purified from the aerial parts of Aristolochia constricta (Aristolichiaceae) on morphine withdrawl in vitro. The results of our experiments indicate that the pure compounds (1-4) significantly and in a concentration-dependent manner reduced the morphine withdrawal. The results of the present study suggest that these new protopine alkaloids may be potential anti-addictive agents. Copyright (C) 2000 John Wiley and Sons, Ltd.