The absorption of ammonium, nitrate, and ammonium and nitrate simultaneously as related to several external factors such as concentration, temperature, and pH, was studied in perennial rye-grass (Lolium perenne L.) Although in general the absorption phenomena of ammonium could be explained in terms of the conventional carrier hypothesis, evidence was obtained that the break-down of the ammonium-carrier complex is dependent on the available amount of some enzyme. In the same way, the absorption of nitrate was shown to fit the carrier hypothesis. It was demonstrated that the absorption of nitrate is followed by an actual accumulation of free nitrate as well as by reduction of this substance, and that both processes take place independently of each other. Whereas the absorption of ammonium was affected by nitrate only to a small extent, the absorption rate of nitrate was considerably inhibited by the uptake of ammonium. Because the accumulation of nitrate was not found to be influenced by ammonium in any way, the effect of ammonium uptake on the rate of nitrate absorption is ascribed to an inhibiting effect of ammonium uptake on the nitrate reduction. As compared to nitrate, both the absorption rate of nitrite and the rate of nitrite reduction proved to be completely independent of ammonium uptake. Therefore, the effect of ammonium absorption on the uptake of nitrate is ascribed to an inhibition of the reduction process of nitrate to nitrite.