Twenty-one alkaloids have already been isolated from the stems and roots of Stephania japonica Miers (Menispermaceae) native to Japan, with basic constituents differing in different habitats. For the purpose of elaboration of chemical studies on Menispermaceous plants, the alkaloids in the leaves from the southernmost plants were investigated. Three known alkaloids, oxostephamiersine (1), 16-oxoprometaphanine (2), and stebisimine (3), were isolated from a methanolic extract of the leaves. Unexpectedly, epistephanine and hypoepistephenine, which were major constituents in the stems and roots, were not found in the leaves. From these findings, it was known that the alkaloids occurring in the leaves are in a more oxidized state than those in the stems and roots, suggesting consideration of metabolic modifications of alkaloids in plant tissues.