The dissertation thesis is focused on the possibility of using dried medicinal plants and pellets as herbal nutraceuticals in the form of supplements in the feeding regimes of ruminants. The aim of this thesis was to observe the effects of medicinal plants on ruminal fermentation processes in vitro and in vivo. For this study, medicinal plants were obtained from commercial sources. A mix of medicinal plants consisted of wormwood (Artemisia absinthium L.), chamomile (Matricaria chamomilla L.), mallow (Malva sylvestris L.), and fumitory (Fumaria officinalis L.). The substrate, which contains barley, meadow hay, and the mixture of medicinal plants (600/300/100 w/w/w) was shown to significantly reduce the production of methane and ammonia during the ruminal and intestinal fermentation in vitro. The presence of polyphenols in the mixture of medicinal plants showed a strong antioxidant capacity in the rumen fluid. In the following experiment, the effect of the mixture of medicinal plants and organic zinc on rumen fermentation, rumen microorganisms, and histopathological changes in lambs in vitro and in vivo was studied. Supplementation of feed with organic zinc (70 mg Zn/kg feed) and a mixture of medicinal plants (100 g dry matter/day/animal) in vitro affected the total amount of gas, methane, and n-butyrate production. A similar effect was not confirmed in vivo, feed substrates did not significantly affect ruminal fermentation parameters or protozoan populations. There was observed a reduction in the total number of bacteria and their shifts in relative abundances in the group supplemented with organic zinc. The interaction of time and type of diet revealed the effect of supplemented feed on the number of neutrophils in the animals' blood. It has also been shown that seventy days supplementation of organic zinc combined with a mixture of medicinal plants can adversely affect the rumen epithelium of animals. Finally, the complete replacement of meadow hay with pellets of sainfoin (Onobrychis viciifolia Scop.) in the feeding regime of lambs experimentally infected with the gastrointestinal nematode Haemonchus contortus reduced the amount of methane by 28 % without adversely affecting other fermentation parameters or ruminal microbial ecosystems. The changes in the tissues of the abomasum of lambs infected with the H. contortus were mild and included mainly epithelial cell damage and inflammatory infiltration. The use of herbal nutraceuticals as natural antioxidants with bioactive properties in ruminant feeds in the form of dried medicinal plants or pellets is proving to be an alternative strategy to common antimicrobials and chemotherapeutics.