Rice seedlings were incubated with barnyard grass root exudates for 10 days, and allelopathic activity of rice extracts was determined by barnyard grass bioassay. The presence of root exudates increased the allelopathic activity of the rice extracts at concentrations greater than 30 mg/L of root exudates, and increasing the concentration increased the activity (Fig. 4). At concentration of 200 mg/L of root exudates, which was equivalent to that obtained from the incubation of 50 barnyard grass, the inhibitory activity of rice extracts against barnyard grass root and shoot growth was 76 and 74%, respectively. The root exudates of barnyard grass also increased momilactone B at concentrations greater than 30 mg/L of root exudates, and increasing the root exudate concentration increased the momilactone B concentration in rice seedlings (Fig. 5). At concentrations of 200 mg/L of root exudates, momilactone B concentration in rice was 6.5-fold greater than that in the control (0 mg of root exudate). There was no significant difference in the osmotic potential between the medium containing barnyard grass root exudates and control medium (all about 10 mmol/kg), and pH value of the medium was maintained at 6.0 throughout the experiments as described in the section of “Materials and methods”. These results suggest that chemical constituents in barnyard grass root exudates may induce the allelopathic activity and momilactone B production in rice seedlings. Thus, the chemical constituents may affect gene expressions involved in momilactone B biosynthesis.