A series of 1-ar(o)yl-3-[2-(1-benzyl-4-piperidinyl)ethyl](thio)urea derivatives was synthesized and evaluated for antiacetylcholinesterase activity. Most aroyl(thio)urea derivatives showed potent inhibitory activity in the sub-micromolar range. A comparable potency was obtained with the aryl(thio)urea analogues by replacing the phenyl with a 2-pyridyl group. The substituted guanidine variations proved to be almost inactive whereas the nitroethylene analogues appeared to be quite efficient. These results were interpreted in terms of the preferential cis-trans conformation of the aroyl(thio)urea and 2-pyridyl(thio)urea moieties involving the existence of hydrogen bonding. In vivo experiments showed that compound 7m had maximal antiamnestic activity at 0.03 mg/kg with a therapeutic ratio greater than 1000, while cholinergic side effects were only seen at doses 100-fold the maximally effective antiamnestic dose. Compound 7m represents a potentially interesting antidementia agent.