In this study, changes in activity of key enzymes of nitrogen assimilation, namely nitrate reductase and glutamate dehydrogenase upon boron (B) toxicity in wheat (Triticum aestivum) and barley (Hordeum vulgare) cultivars were investigated. Ten-day-old seedlings were exposed to 10 mM boric acid for a duration of five days. Plants growing on the nutrient solutions and receiving no excess B, were maintained as controls. All experiments were conducted on leaf and root tissues of control and B-treated seedlings of B-tolerant and B-sensitive cultivars. For estimation of activity of nitrate reductase, an in vivo assay was used. Compared to controls (no B treatment) activity of nitrate reductase tended to decrease (15-17%) following B toxicity in root and leaf tissues of all cultivars, however, no significant difference was observed between resistant and sensitive cultivars. Boron stress increased activity of glutamate dehydrogenase in roots and leaves of all cultivars by an average of 81% and 30%, respectively. Compared to sensitive variety, the boron tolerant wheat variety exhibited a significantly higher increase in shoot tissue GDH activity, whereas no significant difference was observed in root tissues. It was concluded that, the increase in activity of glutamate dehydrogenase could be an adaptive mechanism in these species and possibly plays a protective role under boron-stress conditions.