Onion (Allium cepa L.) is one of the most widely cultivated species of the genus Allium (Amarylldaceae) which comprises over 500 species. The distinctive taste, smell as well as frequent lacrimation and pharmacological benefits are due to organosulphuric compounds formed by enzymatic hydrolysis of nonvolatile S-alk(en)yl cysteine sulphoxides. In this work, the isolation of essential oils was performed by hydrodestillation in Clevenger type apparatus from commercially obtained samples (yellow, red and white onion) as well as by dichloromethane extraction after the autolysis for 24 hours. Non-volatile residue in the flask, collected after the hydrodestillation, was filtered and lyophilized. The samples were tested on the antioxidant activity using three methods (DPPH, FRAP and Rancimat) and acetylcholinesterase and butyrylcholinesterase inhibition (by Ellman method). Inhibition of these cholinesterases still represents the only pharmacotherapy against Alzheimer's disease, which is responsible for more than 50% of neurological diseases among persons over the age of 65 years. The antioxidant activity of all tested samples was very low at tested concentration of 227 µg/mL (<15%). Contrary to the volatile dichloromethane extracts and nonvolatile water extracts, the essential oils showed good cholinesterase inhibition. GC-MS analyses showed that the essential oils contained a high percentage of dipropyl disulphide (up to 50%) and dipropyl trisulphide (up to 20%) in all three varieties, while volatile dichloromethane extract had none of the mentioned compounds. Due to its low molecular weight and lipid solubility these volatile sulphur compounds have ability to pass the blood-brain barrier, and thus represent an interesting class of compounds in treatment of Alzheimer and other neurological diseases.