The aim of the experiment was to evaluate the effect of mineral fertiliser on the above- and under-ground biomass production of permanent grassland. The experiment site was located in mountain production area at an altitude of 960 m. In the grassland of Festuco-Cynosuretum association, 3 fertiliser treatments (PK, N90+PK, N180+PK) were compared with the control (no fertilisation) over 1992-1998. To assess primary production, sward was cut three times and biomass root samples were also taken at the date of cutting. The positive effect of mineral fertilization on an increase in the above-ground dry matter (DM) production was confirmed. Mean DM production has increased by 68.6%, 154.2% and 224.9% in the PK, N90 and N180 treatments, respectively, in comparison to the N0 treatment. There were statistically significant differences between treatments. The amount of under-ground biomass increased moderately by fertilisation. However, the effect of fertilisation on the under-ground biomass was not significant. At the same time changes in an R:S ratio were observed. The widest ratio was found in the control (1.58) and the narrowest one in the N180 treatment (0.52). An R:S ratio in the N0 treatment was lower by 40.5%, 57.6% and 67.1% compared with the PK, N90 and N180 treatments.