Biomass accumulation, species richness, nitrogen availability and its distribution in vegetation were followed in an acidophilous dry grassland and in stands dominated by tall grasses expanding into the original sward. Stands with tall grasses produced and accumulated significantly more of total aboveground plant matter (by 349 gm-2 in Arrhenatherum elatius and by 707 gm-2 in Calamagrostis epigejos) and of total belowground dry mass (by 593 gm-2 in Arrhenatherum elatius and by 954 gm-2 in Calamagrostis epigejos) than the original dry grassland, where the respective dry mass reached only 523 and 1112 gm-2. The differences are probably due to a more effective utilization of nitrogen, since the nitrogen productivity was in C. epigejos 1.2 times and in Arrhenatherum elatius even 1.9 times as great as in dry grassland (91.3 ggN-1year-1). Thus these tall grasses require probably lower amount of nitrogen for the formation of the same aboveground biomass. The greatest accumulation of nitrogen was in total aboveground plant matter and total plant parts in Calamagrostis epigejos. Negative correlations were found between increasing dominance of tall grasses and species richness, which, on the average, decreased in Arrhenatherum elatius and Calamagrostis epigejos to 63% and 44% of the original number in dry grassland. Data on soil N-mineralization rate indicate that this process was 5 times as intensive in stands dominated by tall grass C. epigejos as in intact dry grassland. The enhanced N supply can thus accelerate the species replacement.