In order to test the stress hypothesis of allelopathy of Reigosa et al. (1999, 2002), the combined action of a well-established allelochemical compound (2-3H-benzoxazolinone, BOA) and a common abiotic stress (salt stress) were investigated in lettuce (Lactuca sativa L.). In a previous study (Baerson et al. 2005), we demonstrated that the primary effects of BOA are related to the expression of genes involved in detoxification and stress responses, which might serve to simultaneously alleviate biotic and abiotic stresses. Through analysis of the same physiological and biochemical parameters previously studied for BOA alone (Sánchez-Moreiras & Reigosa 2005), we observed specific effects of salt stress alone, as well as for the two stresses together (BOA and salt). This paper demonstrates that plants showing tolerance to salt stress (reduced stomatal density, increased proline content, higher K(+) concentration, etc.) become salt sensitive (markedly low Psiw values, high putrescine content, increased lipid peroxidation, etc.) when simultaneously treated with the allelochemical BOA. We also report additional information on the mechanisms of action of BOA, and general stress responses in this plant species.