Rhizoma Chuanxiong (Ligusticum chuanxiong HORT.) is a commonly used traditional Chinese medicinal herb for the treatment of cardiovascular disorders. Significant variations of the main components in this herb were observed in commercial samples. The present study investigated effects of post-harvest drying and processing methods on nine main components in the herb. Results showed that drying at 60°C or under the sun the contents of three major constituents, namely senkyunolide A (4), coniferylferulate (5) and Z-ligustilide (6), decreased significantly, while the contents of ferulic acid (1), riligustilide (8) and levistolide A (9) increased significantly. Senkyunolide I (2) and senkyunolide H (3), which were not detected in fresh herbs, appeared in dried samples. Similar chemical alterations, such as decrease in the contents of three major ingredients and increase in the contents of compounds 1, 2, 3, 8 and 9, were also observed in differently processed herbal samples. The possible converting mechanisms of these components were clarified by employing pure major components treated under the same conditions. © 2007 Pharmaceutical Society of Japan.