Sorghum cv. Pioneer 93 1, sorghum-sudangrass hybrid cv. Sioux and maize plant cv. Blizzard were assayed for toxic concentrations of nitrate-nitrogen ($NO_3$-N) and their relationship to morphological characteristics and environmental temperature in a field and phytotron trial. In the phytotron, sorghum and maize plants ranging from emergence to heading stage, were grown under different day/night temperatures of 30125, 25/20,28/18 and 1818 degree C. Nitrate-nitrogen in sorghum and maize plants was accumulated mainly in stems. Therefore nitrate concentration in the young plants was increased as development of stalks advanced and was highest at the stage of 3-4 leaves, when the plants had a leaf weight ratio 0.78-0.80 g/g plant weight. However, nitrate concentrations of the plant decreased as morphological development progressed, especially from the stage of growing point differentiation. Correlation coefficients showed a positive correlation of nitrate concentration with leaf weight ratio, leaf area ratio and specific leaf area, while plant height, dry matter percentage and absolute growth rate showed a negative association with TEX>$NO_3$</TEX>-N ($P{le}0.1$%). Cyanogenic glycosides, total nitrogen and crude protein were close associated with nitrate accumulation, and positively significant ($P{le}0.1$%). High temperature over 30/25^{circ}C.$ for 3 weeks increased N-uptake and dry matter accumulation, but reduced nitrate concentration. Under cold temperature below 18/8^{circ}C.$ concentration of nitrate-N was increased in spite of its limited nitrogen uptake and plant growth.