Acquisition of resistance to lethal concentrations of octanoic acid was induced in cells of Saccharomyces cerevisiae grown in the presence of sublethal concentrations of this lipophilic acid or following rapid exposure (1 h) of unadapted yeast cells to mild stress imposed by the same acid. Experimental evidence indicated that the referred adaptation involved de novo protein synthesis, presumably due to the rapid induction of a plasma membrane transporter which mediates the active efflux of octanoate out of the cell. Rapid exposure of cells to mild ethanol stress also led to increased resistance to lethal concentrations of octanoic acid. This cross-resistance to octanoic-acid-induced death was below the level of resistance induced by mild octanoic acid stress and did not involve induction of the active expulsion of octanoate out of the cell. However, the rapid exposure of yeast cells to octanoic acid or ethanol led to the activation of plasma membrane H+-ATPase. The physiological role of the two stress responses examined during the present study, namely, the active efflux of octanoate specifically induced by octanoic acid and the stimulation of plasma membrane H+-ATPase activity, is discussed.