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-Medicago sativa L. var. Resis was harvested at two successive stages during its first growth cycle and at the end of the third cycle: lucernes 1, 2 and 3. These three samples were submitted to an industrial dehydration treatment with or without prior addition of urea (30 g kg-1 DM). The effect of adding urea was studied: i) on nitrogen content, cell wall composition and in vivo organic matter (OMd) and cell wall (CWd) digestibility measured in sheep using the three dehydrated lucernes; and ii) on ruminal degradation and intestinal digestibility of nitrogen (N) and amino acids (AA) of dehydrated lucerne 1. Urea treatment increased significantly (P < 0.05) the nitrogen content of the three lucernes (+ 4 to 13 g kg-I DM). Nitrogen gain comes mainly from remaining urea-N. Urea treatment decreased the NDF content of lucerne 3 only (-37 g kg-I DM). This treatment improved the OMd (2.9 to 3.7 points) of the three lucernes and the CWd of lucernes 2 (+3.1 points) and 3 (11.7 points). It tended to increase the effective ruminal degradability of nitrogen of lucerne 1 (+ 2.1 points taking into account the microbial contamination). This increase was essentially due to that of nitrogen being rapidly solubilized in the rumen, some of which coming from the nitrogen supplied by the treatment. Correlatively, the urea treatment increased the 16-h ruminal degradation of the AA (+ 5.2 points for total AA), but no effect on intestinal digestibility was observed. However, this result requires confirmation, since a probable excessive heating suffered by untreated lucerne I may have overprotected the AA from ruminal degradation, which could explain the difference from treated lucerne I. In that case, the urea treatment would not affect the PDIA content of lucerne I. The corrected intestinal digestibility value of total AA (0.86) was appreciably higher than that (0.70) (4 à 13 g kg-1 MS). Le gain d'azote provenait surtout de l'azote restant sous forme uréique. Le traitement à l'urée n'a diminué la teneur en NDF que pour la luzerne 3 (-37 g kg-' MS). L'addition d'urée avant la déshydratation a entraîné une amélioration de la digestibilité de la MO (2,9 à 3,7 points) mais n'a augmenté celle des parois que pour les luzernes 2 (+ 3,1 points) et 3 (+ 11,7 points). De plus, le traitement à l'urée tend à augmenter la dégrada-tibilité théorique de l'azote dans le rumen de la luzerne 1 (+ 2,1 points après déduction de la contamination microbienne). Cette augmentation est due essentiellement à celle de l'azote rapidement solubilisé dans le rumen dont une partie provient de l'azote apporté par le traitement. Le traitement à l'urée a augmenté la dégradation ruminale à 16 heures des AA (+ 5,2 points pour les AA totaux) sans toutefois affecter leur digestibilité intestinale. Cependant, ce résultat provenant de l'analyse d'un seul échantillon a besoin d'être confirmé car le chauffage vraisemblablement excessif subi par la luzerne 1 non traitée peut avoir surprotégé les acides aminés de la dégrada-tion ruminale, ce qui expliquerait la diffé...