An in vitro investigation using the rumen simulation technique (Rusitec) was carried out to investigate the effects of dietary supplementation of plant preparations with known antimicrobial properties on ruminal fermentation. The following preparations were added to a mixed forage-concentrate (1:1) diet (g/kg feed dry matter): Hypericum perforatum flowers (2.86), essential oils of Pinus mugo (0.57) and a plant extract mixture (5.71) containing these two plant preparations in proportions of 0.5 and 0.1 of total, respectively. None of the treatments significantly affected ruminal fermentation traits. A trend towards enhanced bacterial population and daily volatile fatty acid formation was observed with the mixture. Ruminal nitrogen turnover showed a positive trend with Hypericum-only supplementation without affecting protozoal population. According to the present results, it is possible to use these plant extracts as alternatives to antibiotics for the lower gut in the dosages tested without adversely affecting the ruminal ecosystem.