<P><B>Abstract</B></P><P>We aimed to manipulate the metabolism of <I>Saccharomyces cerevisiae</I> to produce lactic acid and search for the potential influence of acid transport across the plasma membrane in this process. <I>Saccharomyces cerevisiae </I>W303‐1A is able to use <SMALL>l</SMALL>‐lactic acid but its production in our laboratory has not previously been detected. When the <SMALL>l</SMALL>‐<I>LDH</I> gene from <I>Lactobacillus casei</I> was expressed in <I>S.?cerevisiae </I>W303‐1A and in the isogenic mutants <I>jen1</I><I>∆</I>, <I>ady2</I><I>∆</I> and <I>jen1</I><I>∆ </I><I>ady2</I>∆, all strains were able to produce lactic acid, but higher titres were achieved in the mutant strains. In strains constitutively expressing both <I>LDH</I> and <I>JEN1</I> or <I>ADY2</I>, a higher external lactic acid concentration was found when glucose was present in the medium, but when glucose was exhausted, its consumption was more pronounced. These results demonstrate that expression of monocarboxylate permeases influences lactic acid production. Ady2 has been previously characterized as an acetate permease but our results demonstrated its additional role in lactate uptake. Overall, we demonstrate that monocarboxylate transporters Jen1 and Ady2 are modulators of lactic acid production and may well be used to manipulate lactic acid export in yeast cells.</P>