The sensitivity of Sporidiobolus ruinenii yeast to the use of reducing agents, reflected in changes in the oxido-reduction potential at pH 7 (Eh(7)) environment, ricinoleic acid methyl ester catabolism, gamma-decalactone synthesis, cofactor level, P-oxidation activity, and ultrastructure of the cell, was studied. Three environmental conditions (corresponding to oxidative, neutral, and reducing conditions) were fixed with the use of air or air and reducing agents (hydrogen and dithiothreitol). Lowering Eh(7) to neutral conditions (Eh(7) = +30 mV and +2.5 mV) favoured the production of lactone more than the more oxidative condition (Eh(7) = +350 mV). In contrast, when a reducing condition was used (Eh(7) -130 mV), the production of gamma-decalactone was very low. These results were linked to changes in the cofactor ratio during lactone production, to the beta-oxidation activity involved in decanolide synthesis, and to ultrastructural modification of the cell.