The methanol extracts from five different plant families (Asteraceae, Euphorbiaceae, Melastomataceae, Rubiaceae, and Solanaceae) collected at Regional Natural Park Ucumarí (Colombia), were screened for their acetylcholinesterase inhibitory activity through the modified Ellman's spectrophotometric method. The best inhibitory activities on this study were shown by the extracts of Solanum leucocarpum Dunal (IC 50 = 204.59 mg/l) and Witheringia coccoloboides (Damm) (IC 50 = 220.68 mg/l), both plants belonging to the Solanaceae family.Key words: acetylcholinesterase (AChE) inhibitors -Asteraceae -Euphorbiaceae -Melastomataceae -Rubiaceae -SolanaceaeThe acetylcholinesterase enzyme (AChE) is an attractive target for the rational drug design and for the discovery of mechanism based inhibitors because of its role in the hydrolysis of the neurotransmitter acetylcholine (ACh). AChE inhibitors are the most effective approach to treat the cognitive symptoms of Alzheimer disease (AD) (Kalauni et al. 2002) and other possible therapeutic applications in the treatment of Parkinson's disease, senile dementia, and ataxia, among others (Ahmad et al. 2003a). AChE inhibitors as eserine, tacrine, donepezil, rivastigmine, and galanthamine are the only drugs currently approved for the treatment of AD; however, these drugs are known to have limitations for clinical use due to their short-half-lives and/or unfavorable side-effects (Sung et al. 2002).Because the search of plant extracts that selectively inhibit AChE is of paramount importance to find novel and more potent AChE inhibitors, many of them have been screened and as consequence there have been found extracts and isolated pure compounds with AChE and/or butyrylcholinesterase (BChE) inhibitory properties. Among the plant families that have been examined for their AChE inhibitory activity are: Amaryllidaceae Continuing with the study of the flora from Regional Natural Park Ucumarí (RNPU, Risaralda, Colombia) as a source of new secondary metabolites with diverse grade of biological activities (Niño et al. 2003, 2006, Mosquera et al. 2004a) and the fact that there still is great interest in finding novel and better AChE inhibitors, prompted us to screen 27 crude methanol extracts belonging to the Asteraceae, Euphorbiaceae, Melastomataceae, Rubiaceae, and Solananceae families for their AChE inhibitory activities. MATERIALS AND METHODSIn this study, the solvent hexanes, dichloromethane, and methanol (analytical-reagent grade) were purchased from Mallinckrodt (Phillipsburg, NJ, US). Silica gel plates (Silica gel 60 F 254 0.2 mm layer thickness) were purchased from Merck (Darmstadt, Germany). The electric eel acetylcholinesterase Sigma (St Louis, MO, US) was used. Acetylthiocholine iodide and 5,5-dithiobis (2-nitro benzoic acid) (DTNB) were purchased from Aldrich (St Louis, MO, US). A Spectronic Genesys 5 spectrophotometer Milton Roy (Rochester, NY, US) was used for all measurements of acetylcholinesterase inhibitory activity.Plant material -Twenty seven plants from the selected famili...