Nitrogen stress inLolium rigidum was measured 5 times during the 8 weeks following emergence. Symptoms of nitrogen deficiency appeared when the stress was greater than 40 per cent. Stresses lower than this were accurately estimated either by the concentration of total nitrogen, or free ninhydrin nitrogen in the youngest fully expanded leaf, or by a leaf elongation index. Each of these estimators could be used for diagnosis of nitrogen stress with a single calibration curve, respectively, over the following minimum periods after emergence: total nitrogen — 6 weeks; free ninhydrin nitrogen −4th to the 8th week; leaf elongation — 4th to the 9th week. Preliminary calibration tables are given for the symptom-free range of nitrogen deficiency.