In a bioassay-guided search for acetylcholinesterase inhibitors from Korean natural resources, four isoquinoline alkaloids, corynoxidine (1), protopine (2), palmatine (3), and berberine (4) have been isolated from the methanolic extract of the aerial parts of Corydalis speciosa. Structures of these compounds were elucidated on the basis of spectroscopic techniques. These compounds inhibited acetylcholinesterase activity in a dose-dependent manner, and the $IC_50$ values of compounds 1-4 were 89.0, 16.1, 5.8, and 3.3 $mu$ M, respectively.