In this paper the combined use of in vivo (13)C-nuclear magnetic resonance spectroscopy and mathematical modelling allowed the analysis of the response to ethanol stress in a wild-type strain of Saccharomyces cerevisiae, in terms of a reduced metabolic activity. The model developed succeeded in describing and interpreting the effects of increasing concentrations of exogenous ethanol. In particular, the ratio between the kinetic constants associated with ethanol production and glucose consumption gave the estimation of the metabolic yield of the processes in perfect agreement with experimental results.