The combined influence of several environmental factors (nitrate concentration, pH, bicarbonate concentration and temperature) on the rate of nitrate absorption was investigated in intact maize roots in water culture. By means of a carrier hypothesis (picture of a revolving belt) the results may be described as follows. No measurable diffusion resistance is present between the carrier system and the environment. The carrier, therefore, acts at the very surface of the epidermal root protoplasm; it has a highly specific affinity to nitrate ions and its loading capacity is influenced by pH between 6 and 8. A ‘solidification’ of lipoid membranes is supposed to inhibit nitrate absorption at temperatures below 10° centigrade.