We examined the pharmacokinetic and pharmacodynamic properties of liquiritin apioside, a main antitussive component of Glycyrrhizae radix (licorice), with regard to its antitussive effect in guinea pigs. The peak plasma concentration of the unchanged compound was observed 15 min after the administration of liquiritin apiosaide. The plasma concentration then gradually decreased and was almost undetectable 4 h after administration. Liquiritigenin, a des-glycoside of liquiritin apioside, appeared in the plasma 2 h after the administration of liquiritin apioside and remained for more than 6 h after administration. The plasma concentration of unchanged liquiritigenin was observed 15 min after administration and then gradually increased for more than 6 h after administration. When the antitussive effects of liquiritin apioside, liquiritin and liquiritigenin, at respective doses of 30 mg/kg, p.o., were examined 1 h after administration, liquiritin apioside and liquiritigenin caused a significant reduction in the number of capsaicin-induced coughs. However, at the same dose, liquiritin had no significant effect on the number of capsaicin-induced coughs. On the other hand, when the antitussive effects of liquiritin apioside, liquiritin and liquiritigenin, at doses of 30 mg/kg, p.o., were examined 4 h after administration, each caused a more than 40% reduction in the number of capsaicin-induced coughs. The present results suggest that G. radix (licorice) may produce a persistent antitussive effect, and that liquiritin apioside plays an important role in the earlier phase, while liquiritigenin, which is a metabolite of liquiritin apioside and liquiritin, plays an important role in the late phase.