Résumé : Three indica rice cultivars (Oryza saliva) differing in their tolerance to salt and drought stress in field conditions in Vietnam were analyzed at the molecular and biochemical levels with the goal to reveal the basis for their differential behavior and in particular their ability to accumulate proline. An in vitro growth test showed that after a 7-day period of stress, the fresh weight of plantlet roots appears to be a relevant parameter for differentiating drought and salt tolerance of the concerned cultivars. Sodium level was lower in the salt tolerant cultivar than in the other rice cultivars. Proline accumulation in roots of tolerant cultivars starts earlier after the initiation of the stress treatment than that of the osmotic stress sensitive cultivar and also reaches a higher level. Proline accumulation was not related to proteolysis and so could be the result from induction of proline biosynthesis by osmotic stress. However, neither the sequence of amino acids involved in the proline feedback inhibition of the key regulatory enzyme Δ 1-pyrroline-5-carboxylate synthetase (P5CS; EC not assigned), nor the expression of the p5cs genes were modified in the tolerant cultivars. These observations suggest that proline accumulation in roots is a possible indicator of the osmotic tolerance in these rice cultivars. However, other mechanisms than those related to a change in P5CS regulation are responsible for the increased proline content. © 2003 Elsevier Ireland Ltd. All rights reserved.