The effect of different respiratory inhibitors on the ergosterol content of microaerobically grown non-proliferating yeast cultures was monitored during adaptation to oxygen. It was found that dinitrophenol, azide, and cyanide, which act on the mechanism of the respiratory chain, cause a marked stimulation of sterol production. Acriflavine and chloramphenicol, which affect the synthesis of the respiratory apparatus, caused a delay in the onset of ergosterol synthesis or a marked decrease in sterol content. The data obtained provide presumptive evidence that a component of sterol formation is synthesized on the 70S ribosomal system of the mitochondrion and induced in the presence of oxygen.