Isolated soybean leaf mesophyll cells decarboxylated exogenously added [1-14C]glycolate and [1-14C]glycine in the dark. The rate of CO2 release from glycine was inhibited over 90% by isonicotinic acid hydrazide and about 80% by KCN, two inhibitors of the glycine to serine plus CO2 reaction. The release of CO2 from glycolate was inhibited by less than 50% under the same conditions. This indicates that about 50% of the CO2 released from glycolate occurred at a site other than the glycine to serine reaction. The sensitivity of this alternative site of CO2 release to an inhibitor of glycolate oxidase (methyl-2-hydroxy-3-butynoate) but not an inhibitor of the glutamate:glyoxylate aminotransferase (2,3-epoxypropionate) indicates that this alternative (isonicotinic acid hydrazide insensitive) site of CO2 release involved glyoxylate. Catalase inhibited this CO2 release. Under the conditions used it is suggested that about half of the CO2 released from glycolate occurred at the conversion of glycine to serine plus CO2 while the remaining half of the CO2 loss resulted from the direct oxidation of glyoxylate by H2O2. The rate of glycine decarboxylation by the glycine to serine reaction was apparently controlled by the amount of NAD in the mitochondria. Mitochondrial electron transport inhibitors, KCN and actinomycin A, inhibited glycine decarboxylation while an uncoupler, 2,4-dinitrophenol, stimulated the reaction. Competition within the mitochondria between the enzymes of dark respiration and glycine decarboxylation for limiting NAD may force substantial amounts of the glycolate formed to be decarboxylated by the direct oxidation of glyoxylate.