In order to isolate growth and germination inhibitors from the hulls of dormant rice seeds, the seeds of a Japonica cultivar 'Koshihikari' and an Indica one 'Surjamukhi', which have strong dormancy, were used. The rice hull (ca 200kg) was soaked in 80% methanol for about 2 months. The methanol extracts were concentrated to an aqueous phase by the flash evaporator at below 40°C. The aqueous phase was extracted with chloroform and the active fraction was moved in chloroform. The active chloroform fraction was successively fractionated by organic solvents : n-hexane, benzene, ether, ethylene chloride, and ethyl acetate. The biological activity of each extract was checked by lettuce germination test. The active fraction checked was further separated according to solubility in dilute acid or base. The neutral fraction of benzene and ether extracts showed a strong inhibitory activity. The active fraction was used for further chromatographic study. By the silica gel thin layer chromatography with a developing solvent, benzene : ethyl acetate (10 : 1), two active compounds were crystallized and were identified as momilactone-A (C20H2603) and -B (C20H26O4) The momilactones inhibited the seed germination and the seedling growth of rice and lettuce at a concentration of 10-3M. Especially, these substances remarkably inhibited the root growth of rice seedling at 10-5-10-3M. Momilactone-B had stronger inhibition than momilactone-A. The inhibitory activity of momilactone-B was similar to that of a typical growth regulator, abscisic acid. 'Koshihikari' contained more momilactone-A than momilactone-B, while 'Surjamukhi, which has stronger dormancy than 'Koshihikari', held more mornilactone-B than momi-lactone-A. It was supposed that the content of momilactone-A and -B in rice seeds might vary with the rice cultivar. The inhibitory activity of momilactones was partially reduced by GA3 or IAA. In considering the breaking of dormacy in rice seeds, it may be neces-sary to check the interaction between momilactones and GA or IAA which has an important role during rice seed germination.