Velvet bentgrass (Agrostis canina L.) varieties are reported to have high nitrogen use efficiency and increased disease resistance. Unfortunately, velvet bentgrass exhibits chlorosis or nitrogen toxicity symptoms at nitrogen rates typically associated with optimal growth of other bentgrass species. Previously presented research has shown that this is correlated to ammonium accumulation when fertilizers with a high ammonium content or urea are applied.  We have examined enzymes involved in nitrogen assimilation pathway to determine why ammonium accumulation may be occurring. The cultivar ‘SR7200’ of velvet bentgrass was exposed to 4 different nitrogen forms including 5% ammonium, 45% ammonium, 95% ammonium, and urea in a hydroponic system.  We then measured nitrate, nitrite and ammonium content in addition to nitrate reductase, nitrite reductase and glutamine synthetase (GS) glutamate synthase (GOGAT) at various time points after exposure to the different nitrogen forms. Nitrate reductase and nitrite reductase could not be correlated to the ammonium accumulation.  Interestingly, GS activity in the leaves of SR7200’ had a sudden decrease in the 95% ammonium- and urea-treated plants at the 24 hour time point. The accumulation of ammonium may be caused by the inability of GS to convert the pools of ammonium into amino acids, resulting in chlorosis of velvet bentgrass plants.