The release levels of a growth inhibitor, momilactone B, from rice (Oryza sativa L.) seedlings of eight cultivars were compared with the endogenous concentrations of momilactone B in their seedlings. All rice cultivars contained momilactone B in the seedlings, and their concentrations differed between the cultivars. Momilactone B was also found in all culture solutions in which these rice seedlings were grown, and the concentrations differed between the cultivars. The momilactone B concentrations in the culture solutions were reflected in the momilactone B concentrations in the seedlings. These results suggest that all rice cultivars may produce momilactone B and release momilactone B into the culture solutions. In addition, the release level of momilactone B may depend on the production level of momilactone B in the seedlings, which may affect allelopathic potential of these rice cultivars because as a growth inhibitor, momilactone B is able to act as an allelochemical.