In vivo and in vitro sublethal exposure of abrin, glycyrrhizin (active component of Abrus precatorius seed and root), sanguinarine, protopine (Argemone mexicana seed) and oleandrin (Nerium indicum leaf) inhibited the acetylcholinesterase, lactic dehydrogenase, acid and alkaline phosphatase activity in the nervous tissue of Lymnaea acuminata. The succinic dehydrogenase activity in the nervous tissue of L. acuminata was increased in in vivo treatment of these molluscicides whereas, with in vitro exposure, these molluscicides caused no significant change in succinic dehydrogenase activity.