Pseudocoptisine, a quaternary alkaloid with benzylisoquinoline skeleton, was isolated from Corydalis Tuber, one of the important medicinal plants in traditional medicine. Pseudocoptisine inhibited acetylcholinesterase (AChE) activity in a dose-dependent manner, and the concentration required for 50% inhibition was 12.8 microM. In further study, the anti-amnesic activities of pseudocoptisine in mice on the learning and memory impairments induced by scopolamine (1.0 mg/kg, i.p.) were examined. This alkaloid (2.0 mg/kg, p.o.) administration significantly reversed cognitive impairments in mice by passive avoidance test (p<0.05). It also reduced escape latencies in training trials and prolonged swimming times in the target quadrant during the probe trial in the water maze task (p<0.05). These results indicate that pseudocoptisine has anti-cholinesterase and anti-amnesic activities that may be useful for cognitive impairment treatment.