The investigation aimed to study the effects of methanol fraction of licorice (FM 100) and glycyrrhizin on prostaglandin synthetase activity, in relation to their analgesic effects. Effects of FM 100 and glycyrrhizin on the activity of prostaglandin synthetase extracted from bull seminal vesicles were examined by the modified method of Takeguchi et al. The analgesic effect of FM 100 was tested in mice by the acetic acid writhing method. FM 100 was administered orally to mice. BSV prostaglandin synthetase activity was inhibited significantly by FM 100 in a dose-dependent manner, whereas the activity was slightly inhibited by glycyrrhizin. Statistically significant analgesic effects were also observed with FM 100. The results suggest that analgesic effect of licorice may be due to the inhibition of prostaglandin synthesis.