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Laboratory analyses using the 1st generation bioassay were conducted in the years 2013-2014 to investigate the allelopathic potential of wateralcoholic and aqueous extracts from dry weight of rhizomes and roots as well as stems and leaves of Solidago gigantea. Analysed acceptors were two weed species, i.e. monocotyledonous Echinochloa crus-galli and dicotyledonous Amaranthus retroflexus. When the acceptors (E. crus-galli and A. retroflexus) reached the 2-leaf stage (BBCH 12) they were sprayed with wateralcoholic and aqueous extracts (at concentrations of 12.5%, 10%, 5% and 2.5%) obtained from the donor, i.e. S. gigantea. Results indicate an inhibitory effect of wateralcoholic extracts from aboveground parts (leaves and stems) of S. gigantea in relation to dry weight of E. crus-galli and A. retroflexus. The volume of dry weight reduction in acceptors was dependent on the concentration of extracts produced from the donor plant S. gigantea. Dry weight of E. crus-galli and A. retroflexus was reduced most effectively by two concentrations: 12.5% and 10%. In turn, aqueous extracts from rhizomes and roots of S. gigantea, irrespective of the applied concentration, caused an increase in dry weight of E. crus-galli and A. retroflexus. Only aqueous extracts produced from leaves and stems of S. gigantea, irrespective of their concentration, reduced dry weight in only E. crus-galli.