The study was carried out during the summers of 1982 and 1983 at Lusignan in the central western part of France. Growth curves were established during 2 or 3 successive summer regrowths by harvesting small plots every week. The effects of water stress on dry matter yield (DM) are presented (Fig. 1) to show the intensity of the drought at each regrowth period. At each harvest date the aerial dry matter (ADM) was divided into stem and leaf components. The leaf/stem ratio (L/S) was then calculated. Nitrogen content (N%) was determined separately on the leaf and stem components and also on the total ADM. The in vitro digestibility of dry matter (IVDDM) and the acid detergent fiber content (ADF) were determined both on total aerial dry matter and on the stem component. The change in the measured variables was expressed either in relation to time or to dry matter accumulation. This second expression allowed us to better compare irrigated and non-irrigated conditions. The US ratio decreased more rapidly with time on irrigated than on non-irrigated lucerne. The evolution of L/S in relation to ADM, however, was the same under the 2 conditions and a common relationship between these 2 variables is proposed : L/S = 1.4 (ADM)-0.56 (Fig. 2). This relationship permits the US value to be predicted directly from dry matter yield whatever the water conditions are. The higher values of L/S observed during the drought at a given time were therefore entirely due to a lower ADM accumulation than when water was non limiting. The N content of ADM decreased less rapidly with time under non-irrigated conditions than under irrigated conditions. But for the same quantity of accumulated dry matter, N content was lower in water-stressed than in well watered lucerne (Fig. 4), indicating a lower level of nitrogen nutrition. The global effect of drought on the forage N content was therefore the result of a negative effect on the N nutrition processes which was more or less compensated by a positive effect due to a lower dilution of N. This phenomenon allows us to explain some contradictory results in the literature which relate either positive or negative effect of water stress on the N content of harvested lucerne forage. The water stress led to an increase in IVDDM and to a decrease in ADF content of the harvested forage (Fig. 7). This effect can be explained by the increase in the L/S ratio and by an increase in the digestibility of the stem component. This increase in the digestibility of stems can be directly related to the decrease of stem dry matter. A general relationship between the IVDDM and the dry matter yield of lucerne forage in natural conditions is proposed as an indicator for predicting forage quality.