For some species, mathematical models have been developed to describe tissue N dilution during crop growth and to estimate the plant N status applying the N nutrition index (NNI), the ratio between the actual tissue N concentration and the tissue N concentration needed to obtain the maximum instantaneous crop growth rate (critical tissue N concentration). The objectives of this work were (i) to obtain the critical N dilution curve (NDC) for annual ryegrass (Lolium multiflorum Lam.) corresponding to late winter and early spring growth, (ii) to compare it with NDCs obtained by other authors, (iii) to estimate the NNI for different levels of N fertilization, and (iv) to determine the apparent N use efficiency (NUE) and its components: N uptake efficiency (NUPE) and N conversion efficiency (NCE). Experiments were established in 1994 and 1995 to evaluate impacts of six N fertilization levels (0-250 kg N ha(-1)) applied at the end of winter. Fertilization significantly increased forage dry matter (DM) and N accumulation in both years. A fitted critical NDC [N-cr (g kg(-1)) = 40.73DM (t ha(-1))(-0.379) ; r(2) = 0.69] presented lower values than a published reference NDC. The NNIs calculated using the critical NDC showed higher values at higher N fertilization levels. The validity of the critical NDC and the usefulness of the NNIs should be tested in further experiments. High NUE values were observed in both years (44.20 and 52.21 kg DM kg(-1) N applied in 1994 and 1995, respectively) although NUPE and NCE values varied between experiments.