Simultaneous and continuous measurements of changes in CO2 and O2 concentrations in glucose-metabolizing yeast suspensions by mass spectrometry enabled a study of the Pasteur effect (aerobic inhibition of glycolysis) in Saccharomyces uvarum and Schizosaccharomyces pombe. A different control mechanism operates in Candida utilis to give a damped oscillation after the anaerobic-aerobic transition. The apparent Km values for respiration of the three yeasts were in the range 1.3-1.8 microM-O2. The apparent Km values for O2 of the Pasteur effect were 5 and 13 microM for catabolite-repressed and derepressed S. uvarum respectively and 7 microM for Sch. pombe. These results are discussed with respect to currently accepted mechanisms for the control of glycolysis.