The aim of this study was to determine the effect of nitrogen (N) fertilization on digestibility, crude protein and metabolic energy content in feed on semi-natural grasslands. Over the course of three years (2002, 2003 and 2004) a research was conducted on the Zagreb Faculty of Agriculture's "Center of Grassland" experimental pasture, on semi-natural grassland that was rotary grazed with sheep. The subject of study was the effect of nitrogen fertilization in quantities of 35 kg ha-1 yr-1 of nitrogen (N35), 100 kg ha-1 yr-1 of nitrogen (N100) and 150 kg ha-1 yr-1 of nitrogen (N150) on the productivity of pastures and nutritional values of available plant mass. The study results showed the effect that the more extensive pasture management measures had on the nutritional value of the available feed. The impact of the application of N on the nutritional value of available mass was found only in the reduced content of crude protein (CP) in fertilization with 35 kg N ha-1 yr-1 in comparison with higher levels of applied N. The digestibility of organic matter (OM) of the feed proved to be the highest in the application of 100 kg N ha-1 yr-1, where it amounted to 734.9 g kg-1 DM. The highest digestibility of the feed's OM was found during the growing season in 2003 (757.3 g kg-1 DM), while in 2002 and 2004, the digestibility of the OM was 702.3 and 741.6 g kg-1 DM respectively. The metabolized energy (ME) content was neither significantly changed by different amounts of N fertilization, nor with respect to the vegetation growing season. Nevertheless, the highest content of ME was determined in fertilization with 100 kg N ha-1 yr- 1 (10 MJ kg-1 DM), and in the growing season in 2003 (10.3 MJ kg-1 DM).