Octanoic acid causes the inhibition of growth, decrease of the biomass yield and yield of ATP, stimulation of the specific production of ethanol in Saccharomyces cerevisiae. The toxicity of this fatty acid is higher in cells grown at temperatures below 25 degrees C, particularly below 15.5 degrees C, as indicated by the enhancement of the exponential constant of growth inhibition by octanoic acid with the decrease of temperature. Moreover, its presence leads to an increase of the minimum temperature for growth. The intracellular pH (pH(i)) of cells harvested in the mid-exponential phase of growth at temperatures within the range 8.5-30 degrees C decreased sharply from 6.8-6.9 down to 5.8-6.2 when octanoic acid is present at concentrations of the undissociated toxic form below 14 mg l(-1). However, with more inhibitory concentrations (up to 44.5 mg l(-1)) the pHi only attained 5.7-6.1. This may result from the decreased synthesis of succinic and acetic acids in octanoic-acid-stressed cells and from the increased buffering capacity of their cytoplasm due to the reduced internal volume. The effect of octanoic acid on intracellular acidification is nearly independent of growth temperature within the range 8.5-30 degrees C and cannot explain the augmentation of toxicity at low temperatures.