The time courses of the glycyrrhizin and isoliquiritigenin glycoside contents in the thickening roots of licorice, Glycyrrhiza glabra L., have been determined. The glycyrrhizin content in 1-year-old roots rapidly increased from October to November, whereas the isoliquiritigenin glycoside content increased up to October. In 3-year-old plants, although the isoliquiritigenin glycoside content rapidly increased from June to July, the glycyrrhizin content did not show any significant increase from May to August. The glycyrrhizin content increased during the senescence of the aerial parts as well as during the early stage of shoot elongation. The incorporation of [14C]mevalonic acid into the glycyrrhizin fraction by the root segments was high in May, June and September, and low in August and winter. These results indicated that the biosynthesis of glycyrrhizin is differently regulated from that of isoliquiritigenin glycoside in the thickening root of G. glabra.