At the global level, the invasion of alien organisms represent the second largest threat to biodiversity. The assumption is that the high allelopathic potential is one of the features that helps invasive plant species to spread to new areas. The high level of aggressiveness in most species is supported by the action of specific chemical compounds - allelochemicals. Allelochemicals are secondary metabolites of plant metabolism that determine relationships between individuals of the species called allelopathy. Many allelochemicals act as "chemical weapon" and can potentially be used as natural herbicides, growth regulators or as a base for their synthesis. Applying allelopathy in agriculture can reduce the use of synthetic pesticides. The study was divided into two parts. In the first part of the research, preliminary identification of allelopathic potential of eight invasive non-native plant species (donor species), from six different plant families, was determined. Donor species were Abutilon theophrasti Med. - velvetleaf (family Malvaceae), Ambrosia artemisiifolia L. - ragweed (Asteraceae), Datura stramonium L. – white jimsonweed (Solanaceae), Xanthium strumarium L. - cocklebur (Asteraceae), Ailanthus altissima (Mill.) Swingle – tree of heaven (Simaroubaceae), Amorpha fruticosa L. - indigo bush (Fabaceae), Reynoutria japonica Houtte. - Japanese knotweed (Polygonaceae), Solidago gigantea Aiton - giant goldenrod (Asteraceae). Three cultivated plant species from three different plant families were used as test species: Avena sativa L. - oats (Poaceae), Brassica napus L. - oilseed rape (Brassicaceae) and Helianthus annuus L. - sunflower (Asteraceae). Aqueous extracts of eight donor species were applied to the seeds of the test-plants. Aqueous extracts were made of whole plants. High allelopathic potential of all species included in investigation was proven. Allelopathic effects are exclusively negative (inhibiting) and intensity depending on the type of donor species and test-species. Allelopathic impact on the percentage of germination in all species is much smaller in relation to the impact on the radicle and shoot length of the test-species. Inhibitory effect on germination was determined only to the extract of tree of heaven applied to the oilseed rape seed. Perennial species have a stronger allelopathic potential than annual species. Tree of heaven is species with the strongest allelopathic potential followed by white jimsonweed, indigo bush, giant goldenrod, ragweed, Japanese knotweed. The smallest allelopathic potential was determined in annual species velvetleaf and cocklebur. The most sensitive test-plant is oilseed rape followed by oats and sunflower. Tree of heaven aqueous extract inhibited radicle length of rapeseed 94.88%, and the length of sprouts 98.91%, which is almost herbicidal activity of extracts. Tree of heaven and oilseed rape are therefore included in the second part of the research in which aqueous extracts are made separately from the roots, stems, leaves and whole plant of tree of heaven. Each of the extracts was used in three different concentrations (total of 12 extracts) and applied to the seed, root and leaf of the oilseed rape. Application to the seeds implies wetting the filter paper in Petri dishes, application of the root implies watering in two different rates and application on leaves implies spray application on previously raised test plants in 2 leaf stage (BBCH 12). Inhibitory allelopathic effects of different intensities were observed, depending on the type and concentration of the extracts and on the method of application. The strongest effect was observed applying the extracts on the seeds, followed by application on the leaves (spraying) and on the roots (watering) of the test-plants. Stimulating effect was not found in any part of the research. Obtained results showed that the tree of heaven contains allelochemicals in all investigated parts (root, stem, leaf). Regardless of the method of application, the strongest effects were identified to the root extracts of tree of heaven where is the highest concentration of potent allelochemicals. Liquid chromatography determined that the concentration of ailanthone, in the literature the most potent allelochemical of tree of heaven, is the largest in the root extracts (0,35 mg/ml), then extracts of the stem (0,15 mg/ml) and finally the leaf (0,12 mg/ml) extracts. Concentration of ailanthone in whole plant extracts is in accordance with determined values in individual organs of tree of heaven (0,19 mg/ml). According to the obtained results, it can be concluded that tree of heaven contains allelochemicals in all investigated parts. The largest source of potent allelochemicals is the root. Action of ailanthon, as the most potent allelochemical, is supported by other allelochemicals located in plant tissues of tree of heaven. The strongest effects were determined by applying extracts on the seeds, then applying on the leaf, and the lowest by applying on the root of the test-species. According to the determined test-species reactions, there is a need for further similar studies, finding plants with high allelopathic potential, identification and isolation of allelochemicals with the objective of their application in crop production. Invasive plant species are a good option for further allelopathy research.