Stinging nettle (Urtica dioica L.) is a medicinal, edible, often neglected plant species with a high content of numerous phytochemicals, especially polyphenolic compounds, thus the high potential for human health. The aim of this thesis was to determine the effects of nutrient solution and harvest (mowing) time on total phenolic, flavonoid, and non-flavonoid content, and antioxidant capacity of stinging nettle leaves from floating hydroponics. A two-factor experiment with stinging nettle was set up according to a randomized block design in three replicates in a floating hydroponics in a greenhouse. The content of polyphenolic compounds (total phenols, flavonoids, non-flavonoids) and antioxidant capacity in fresh nettle leaf cultivated in two different nutrient solutions were monitored, during three mowing periods. The content of polyphenolic compounds (total phenols, flavonoids and neflavonoids) and antioxidant capacity in fresh nettle leaf grown in two feed solutions of different nutrient composition, over three mowing periods were monitored. The results of the study showed that repeated mowing and the composition of the feed solution significantly affect the content of polyphenolic compounds and antioxidant capacity. The values of the content of total phenols, flavonoids, neflavonoids and antioxidant capacity values were found to be highest in the third mowing period (total phenols 419.22 mg GAE/100 g fresh matter, flavonoids 214.06 mg GAE/100 g fresh matter, neflavonoids 205.16 mg GAE/100 g fresh matter, antioxidant capacity 2475.76 μmol TE/L). The highest content of total polyphenole compounds and antioxidant capacity, depending on the composition of the feed solution, was established when growing in a solution according to the Johnson recipe, which cumulatively contains a lower nitrogen content compared to the solution according to the Larson recipe. Based on all of the above, it can be concluded that the composition of the feed solution as well as the mowing period are significant factors in the cultivation of nettle in floating hydroponics and that optimal balancing of them can also significantly affect the nutritional quality as a final product.