At present, cholinesterase inhibitors are used in the treatment of memory impairment diseases. Rauvolfia serpentina (L.) Benth. ex. Kurz (RS) has been used as additional herb in some Thai traditional antirejuvenating and neurotonic remedies to potentiate hypnotic and tranquil effects within limited dose of dried root 50mg t.i.d. or 150mg/day. Dried root of RS (20 gm) was macerated in ethanol (48 hr x2 times) and the filtrate was freeze-dried to yield crude extract (RsC) 500mg. Another RS (200 gm) was extracted for alkaloids by partition basic extract fraction with chloroform to obtain crude alkaloids, A (RsA) 345mg and B (RsB)197mg. Determination by TLC then detection under UV 365 and Dragendoff's reagent revealed that RsA was composed of 4 weak-base alkaloids whereas RsB was 2 alkaloids. Enzyme inhibitory activity of both acetylcholinesterase (AChE) and human butyrylcholinesterase (BuChE) were performed on RsC, RsA, RsB by the method of Elman (1,2,3). The results showed that the ethanolic extract of RsC, RsA, RsB at dose of 0.1mg/ml had inhibitory effect more than 70% on both enzymes. The IC50 values on BuChE were lower than on AChE. Moreover, RsB, RsA had more inhibitory activity on both enzyme than RsC. These results revealed the benefit effect of Rs in increasing neurotransmitter when given in limited dose.