Aims: The objective of this work was to examine adaptative responses occurring in Saccharomyces cerevisiae following exposure to the herbicide 2-methyl-4-chlorophenoxyacetic acid (MCPA). Methods and Results: The exposure of a yeast cell population to MCPA concentrations of moderate toxicity led to a period of latency before eventual resumption of inhibited growth. During this period of adaptation, the plasma membrane (PM) H+-ATPase was activated, in coordination with the decrease of intracellular pH (pH(i)), cell viability and average cell volume. The in vivo activation of this ATPase was demonstrated either by assaying PM-ATPase activity in membrane suspensions extracted from cells grown in the presence or absence of MCPA or by measuring the in vivo H+-pumping activity in the same cells. The PM-H+-ATPase activation could not be attributed to transcriptional activation of the encoding genes PMA1 and PMA2. Conclusions: The activity of PM-H+-ATPase was stimulated and the internal cell volume decreased during yeast adaptation to growth under MCPA stress. Based on the values estimated for the pH(i), we hypothesize that these cell responses may contribute to the restoration of pH(i) homeostasis during recovery from MCPA stress. Significance and Impact of the Study: This work is a contribution to the understanding of the toxic effects of the herbicide MCPA and of physiological mechanisms underlying adaptation to MCPA, in the eukaryotic model S. cerevisiae. Results may be useful to elucidate the adaptation mechanisms to this xenobiotic compound in more complex and experimentally less-accessible eukaryotes. They also provide indications to assist the use of yeast cells as a bioassay system to assess the toxicity of phenoxyacetic acid herbicides and of other lipophilic xenobiotics, aiming at reducing the use of animals in toxicity testing.