In the present work physiological and biochemical aspects of boron toxicity in Turkish wheat and barley cultivars were investigated. Studies were conducted on wheat cultivars (Triticum aestivurri) named Bolal (boron tolerant) and Atay (boron sensitive) and barley cultivars (Hordeum vulgäre) named Anadolu (B tolerant) and Hamidiye (B sensitive). Boron stress was applied as 10 mM H3BO3 on 10 days old seedlings for 5 days. For understanding the mechanism that contribute to B-toxicity-tolerance the following experiments were conducted: Firstly the changes in protein profiles of root and shoot tissues of wheat and barley cultivars were investigated by using 2-D-SDS PAGE electrophoresis. Boron stress caused quantitative and qualitative changes in protein patterns of both species. A newly synthesized protein Mwt of 35 kD pi: 7.8 in root of boron tolerant barley cultivars and a novel protein in root of B-tolerant wheat cultivar Mwt of 27 kDa pl:7.1 were detected. Leaf tissue electrophoretic patterns of B stressed seedlings revealed induction of numerous proteins upon boron toxicity especially in tolerant cultivars. It suggests the possibility that these proteins might have a role in B resistance mechanism in these species. Secondly the changes in cell wall uronic acidcontents upon boron toxicity were studied. The results suggested that cell wall uronic acid content does not contribute the detoxification of excess boron in wheat and barley. At the same time the effect of boron on root and shoot proline contents were determined. For all of the wheat varieties tested proline accumulation was observed both in root and shoot tissues. However in barley under boron stress decrease was observed both in boron tolerant and sensitive varieties in both tissues. Changes in photosystem II activities were also measured under B stress. There were decrease in PS-II activities of both species and the decrease being drastic in sensitive ones. The changes in activities of key enzymes of nitrogen assimilation namelly nitrogen reductase and glutamate dehydrogemase and SOD which is the most important enzyme of stress defense mechanism were studied under B stress both in root and shoot tissues of wheat and barley. Total activity of nitrate reductase tended to decrease following boron toxicity in root and shoot tissues of all cultivars and this decrese was more pronounced in sensitive cultivars. Reduction of total activity of NR in root was more than in leaf tissues. Total activity of glutamate dehydrogenase increased upon boron stress in root and leaf tissues of both species. There were significant increase in the SOD activities of the tolerant varieties.