The regulation of the hemoproteins catalase T, catalase A and iso-1-cytochrome c was studied in the yeast Saccharomyces cerevisiae. Levels of catalase T and catalase A mRNAs are low or undetectable in anaerobic and heme-deficient cells, and in wild type strains grown on high glucose concentrations. Regulatory mutants (cgr4 and cas1), which have previously been shown to have high catalase T activity when grown in the absence of oxygen or on high glucose concentrations, have high levels of catalase T mRNA when grown under glucose repression conditions. Whereas no catalase T mRNA could be detected in a heme-deficient (ole3) single mutant, double mutants (ole3 cgr4) and (ole3 cas1) contain mature catalase T mRNA. Catalase T and A mRNAs are accumulated rapidly during adaptation of anaerobic cells to oxygen. Anaerobic and heme-deficient cells lack or have extremely low levels of iso-1-cytochrome c mRNA, which, like catalase mRNAs, is accumulated rapidly during oxygen adaptation. The results obtained demonstrate that glucose, oxygen and heme regulate the synthesis of the hemoproteins studied by controlling mRNA levels. In addition, posttranscriptional, probably translational control has to be postulated at least in the case of catalases, to explain the results obtained.