Climate changes are supposed to influence productivity and chemical composition of plants. In the present experiments, it was hypothesised that the incubation of plants exposed to elevated atmospheric carbon dioxide concentrations ([CO₂]) and drought stress will result in different ruminal fermentation pattern and microbial diversity compared to unaffected plants. Maize plants were grown, well-watered under ambient (380 ppm CO₂, Variant A) and elevated [CO₂] (550 ppm CO₂, Variant B). Furthermore, each CO₂ treatment was also exposed to drought stress (380 ppm and 550 ppm CO₂,Variants C and D, respectively), which received only half as much water as the well-watered plants. Plant material from these treatments was incubated in a semi-continuous in vitro fermentation experiment using the rumen simulation technique. Single strand conformation polymorphism (SSCP) analysis was conducted for Bacteria and Archaea specific profiles. The analysis of crude nutrients showed higher contents of fibre fraction in drought stress Variants C and D. Crude protein content was increased by drought stress under ambient but not under elevated [CO₂]. Fermentation of drought stress variants resulted in significantly increased pH values, decreased digestibilities of organic matter and increased ammonia-N (NH₃-N) concentrations compared with well-watered variants. Additionally, the 550 ppm CO₂ Variants B and D showed significantly lower NH₃-N concentrations than Variants A and C. The Bacteria- and Archaea-specific SSCP profiles as well as the production rates of short-chain fatty acids and their molar percentages were not affected by treatments. During the first four days of equilibration period, a decrease of molar percentage of acetate and increased molar percentages of propionate were observed for all treatments. These alterations might have been induced by adaptation of the in vitro system to the new substrate. The rumen microflora appeared to be highly adaptive and could cope with altered contents of crude nutrients in plants as induced by elevated [CO₂] and drought stress.