In this study, the effects of body and root extracts of "Sideritis libanotica" plant over acetylcholinesterase (AChE: E.C.126.96.36.199) enzyme were investigated. Sideritis libanotica has been traditionally used to aid digestion, strengthen the immune system and suppress common cold, the flu, allergies and shortness of breath, sinus congestion, and even pain and mild anxiety. AChE catalyzes the hydrolysis of acetylcholine, a neurotransmitter substance which functions in certain parts of nervous system. When acetylcholine is hydrolyzed by AChE, the passage between the nerves ends. In diseases related to memory loss, it has been detected that acetylcholine is decomposed in a very short time. It has also been observed that inhibiting the enzyme that breaks down acetylcholine strengthens the passage between the nerves. In this study, human plasma and erythrocytes were used as enzyme sources and they were treated with methanol and hexane extracts of the plant's body and roots. Using acetylthiocholine iodide (AChI) as a substrate and dithiobisnitrobenzoic acid (DTNB) as a coloring agent, changes in esterase activity were spectrophotometrically measured. The results indicate that some forms of body and root extracts from the plant inhibit plasma and/or erithrocyte AChE. 150 values (concantrations where enzyme activity drops to %5o) for plant bodies' methanol and hexane extracts over plasma AChE activity was 1.066x10(-3) and 5.888x10(-3), respectively, while plant root's methanol extract's was 0.174x10(-3). Plant root's hexane extract had no inhibition effect on plasma AChE. On erithrocyte AChE levels, I-50 values for plant's body and root methanol extracts were 0.1679x10(-3) and 0.2136x10(-3), respectively. Neither body nor root hexane extracts had inhibitory effects on erithrocyte AChE levels.