In this work we studied the allelopathic effects of aqueous roots extracts of 11 cvs of sunflower on mustard and wheat germination. We also checked whether soil, after sunflower cultivation, contained compounds of allelopathic activity against mustard plants, and if, roots of sunflower seedlings exude substances of allelopathic activity on co-cultured mustard seedlings. Compounds contained in aqueous extract of roots possessed low (mustard) if any (wheat) allelopathic - inhibitory effect on seed germination. Allelochemicals derived from sunflower roots differ in the mode of action on dynamics of germination being inhibitory in mustard but both inhibitory and stimulatory in the case of wheat. Contrary, mustard plant growth was strongly inhibited by compounds that were in the soil, previously used for sunflower cultivation, most probably root exudates and not possible to collect root parts. Growth of mustard seedlings cocultured sunflower seedlings was also inhibited most possibly due to exudation, by sunflower roots, substances of allelopathic activity. Inhibitory effects of sunflower on mustard growth were more pronounced on mustard hypocotyls and shoots than on roots.