The interactions between plants using chemical substances secreted by them are called allelopathy. Positive allelophatic impact may have the practical aspect in order to protect against pests, weeds and diseases. The studies evaluated the effects of water extracts of the Agrostemma githago seeds on germination and initial growth of winter cereal species (wheat, barley). In a laboratory experiment in Petri dishes sown grain and dealt with relevant extracts (1, 2, 5 or 8%). Control plates were watered only with distilled water. Germination, length and fresh mass of roots and cotyledons were analyzed statistically. With the increase in concentration of water extracts of dry matter of Agrostemma githago seeds, a germination decreased. Extracts with a concentration of 5 and 8% inhibited the most the germination of wheat. In the case of barley, such action has been characterized by an extract of 1%. Length of first leaf and roots of germ was significantly lower in the case of both cereals irrespective of the concentration of the extract of the weeds seeds, which implies a significant reduction in weight of seedlings.