The absorption of ammonium and nitrate by perennial rye-grass was studied. Ammonium absorption follows a carrier hypothesis and depends on enzyme availability. Nitrate absorption also fits the carrier hypothesis and involves both accumulation and reduction processes. Ammonium uptake slightly affects nitrate absorption, but nitrate uptake is significantly inhibited by ammonium. Ammonium absorption does not impact nitrite absorption or reduction. The inhibition of nitrate reduction by ammonium uptake affects the rate of nitrate absorption.