Grasslands are rarely irrigated, thus water deficits often induce a reduction of the nitrogen nutrition index (NNI) during summer. This is measured using the ratio between the actual N concentration and the minimum N concentration required to achieve the maximum growth rate. NNI is derived from the standing biomass by a simple relationship. This paper details the results of a field experiment, combining 2 levels of irrigation with 2 levels of nitrogen fertilisation during the summer, on 2 commonly cultivated grass species in pure swards (tall fescue, Festuca arundinacea L., and Italian ryegrass, Lolium multiflorum). Plant water status, NNI, root length density (RLD), soil volumetric water content (theta(v)), and mineral nitrogen concentration [N] were followed under water deficit and recovery. In both species, RLD was high (>6 cm/cm(3)) in the 0-0.25 m soil layer. Whereas the NNI of tall fescue responded strongly to its water status, Italian ryegrass was most often above optimal nitrogen nutrition because of its slow growth in that particular season and its higher superficial RLD. However, its NNI generally followed the theta(v) closely, whereas tall fescue exhibited a delay in response of NNI upon rewatering, suggesting lasting effects of water deficits on the absorption capacity of roots in that species.