In higher plants, MAP kinase cascades are involved in the transduction of numerous stress-related signals but much less is known about the effect of mitogenic signals. We have analysed MAP kinase activation in tobacco cells after treatment by auxin, a growth factor required at physiological concentrations for mitosis in plant cell cultures. From in-gel assay of myelin basic protein kinase and from immunochemical detection of ERK related kinases, we show that the mitogenic effect of auxin, which was confirmed by the specific increase of several mRNAs species, did not rely on MAP kinase activation within the first 2 hours. These data contest previous results which could be due to the activation of MAP kinase by a signal other than auxin. In the second part of this study, we show that the treatment of the cells with high concentrations of various weak lipophilic acids such as auxin, in a non-physiological concentration range, butyric or acetic acid is sufficient to activate transiently a MAP kinase. The data show that MAP kinase activation is the consequence of cytosolic acidification. Moreover, it is not sensitive to the protein kinase inhibitor staurosporine. These results suggest a functional role for cytosolic acidification as a second messenger mediating MAP kinase activation in the response of plant cells to various stresses.