The conductance change due to growth of Saccharomyces cerevisiae Y112, Zygosaccharomyces bailii M and Rhodotorula rubra NCYC 63 in culture media containing glucose, tartrate pH buffer and ammonium ions as sole nitrogen source was compared with that in a medium containing L-asparagine as sole nitrogen source. Decreases in conductance were observed in glucose-ammonium cultures of all three yeasts while little change occurred in cultures with L-asparagine as sole nitrogen source. This supports the hypothesis that the metabolic activity primarily responsible for conductance change in yeast cultures is the uptake of charged ammonium ions as nitrogen source and the reaction of protons with pH buffer compounds. Rhodotorula rubra cultures with L-asparagine as sole carbon source caused large increases in conductance with growth. Chemical analyses of culture filtrates showed that this increase in conductance was due to use of L-asparagine as carbon source and the excretion of nitrogen surplus to biosynthetic needs as ammonium. In addition, the production of aspartate, acetate and bicarbonate contributed to the increase in conductance.