The inhibitory of Corydalis tuber (Corydalis turtschaninovii Besser forma yanhusuo Y. H. Chou et C. C. Hsu) was tested on crude rat lens aldose reductase, an enzyme involved in the complications of diabetes. The methanolic extract (CM-ext) inhibited aldose reductase, while the aqueous extract (CA-ext) was ineffective. Only dehydrocorydaline, of the seven alkaloidal components isolated from CM-ext, inhibited aldose reductase. It is suggested that the inhibitory effect of CM-ext on aldose reductase may be partially attributed to dehydrocorydaline.