Culture solutions in which barley had been grown outside the greenhouse, were more acid than similar solutions in which barley plants had been grown inside the greenhouse. This phenomenon was associated with temperature differences and not with variations in light intensity. Analyses of culture solutions showed both cation and anion absorption to decrease with a lowering of temperature, as the result of a general reduction in the metabolic activity of the plants. Absorption of NO/sub 3//sup -/, however, was affected much more by temperature changes than other ions; almost no NO/sub 3//sup -/ absorption occurred at 13 C. The decrease in NO/sub 3//sup -/ - absorption at the lower temperature was so much greater than that observed for any other ion that a secondary mechanism is suggested as the limiting factor in NO/sub 3//sup -/ absorption: the reductase enzyme system which is essential to the reduction of nitrate and the synthesis of organic nitrogen compounds, is temperature-dependent and reacts very slowly at 13 C. This may interfere with the assimilation of nitrate, and the accumulation of nitrate in the tissues reduces its further absorption. 13 references, 5 figures, 2 tables.