¹Department of Biochemistry, Faculty of Pharmacy, Hacettepe University, Ankara, Turkey DOI : 10.29228/jrp.134 Anti-hormone therapy mediated cognitive decline among cancer patients has been referred to as chemobrain. Current literature indicates that declined estrogen in the brain may be contributing to the genesis of this phenomenon. Aromatase is a key enzyme that converts C19 steroids to estrogen in the brain, although its involvement in cognitive function has remained obscure. This study evaluated the downregulation of aromatase by siRNA treatment and the effect of aromatase downregulation on mRNA expression and specific activity of acetylcholinesterase in SHSY5Y cells. short interfering RNA duplexes were employed to achieve aromatase gene downregulation in SH-SY5Y cells. The reduction of aromatase mRNA was analyzed by real-time PCR. Estradiol levels were determined to confirm the downregulation by a commercial ELISA kit. Acetylcholinesterase mRNA level of siRNA-treated SH-SY5Y cells was analyzed employing Real-time PCR. Specific acetylcholinesterase activity was determined using the Ellman method. Viability and caspase 3/7 activity was examined using ApoTox-GloTM Triplex Assay. zVad-Fmk was employed as a pan-caspase inhibitor in experimental groups.Real-time PCR analysis showed a significant decrease in aromatase mRNA. The downregulation data was also confirmed using an ELISA estradiol kit. Acetylcholinesterase mRNA level was determined significantly reduced in siRNA-treated SH-SY5Y cells. It was observed that the specific activity of acetylcholinesterase was also reduced. Aromatase downregulation didn’t alter viability or caspase 3/7 activity of SHSY5Y cells. However, aromatase downregulation increased susceptibility to caspase-dependent apoptosis.Considering our findings, there may be interactions between aromatase and acetylcholinesterase that have not yet been elucidated and may contribute to the mechanism of chemobrain. Keywords : Aromatase inhibitor; Acetylcholinesterase; chemobrain; siRNA; estrogen