Metabolism of glycine in isolated mitochondria and protoplasts was investigated in photosynthetic, etiolated (barley and pea leaves) and fat-storing (maize scutellum) tissues using methods of [1-(14)C]glycine incorporation and counting of 14CO2 evolved, oxymetric measurement of glycine oxidation and rapid fractionation of protoplasts incubated in photorespiratory conditions with consequent determination of ATP/ADP ratios in different cell compartments. The involvement of different paths of electron transport in mitochondria during operation of glycine decarboxylase complex (GDC) was tested in different conditions, using aminoacetonitrile (AAN), the inhibitor of glycine oxidation in mitochondria, rotenone, the inhibitor of Complex I of mitochondrial electron transport, and inhibitors of cytochrome oxidase and alternative oxidase. It was shown that glycine has a preference to other substrates oxidized in mitochondria only in photosynthetic tissue where succinate and malate even stimulated its oxidation. Rotenone had no or small effect on glycine oxidation, whereas the role of cyanide-resistant path increased in the presence of ATP. Glycine oxidation increased ATP/ADP ratio in cytosol of barley protoplasts incubated in the presence of CO2, but not in the CO2-free medium indicating that in conditions of high photorespiratory flux oxidation of NADH formed in the GDC reaction passes via the non-coupled paths. Activity of GDC in fat-storing tissue correlated with the activity of glyoxylate-cycle enzymes, glycine oxidation did not reveal preference to other substrates and the involvement of paths non-connected with proton translocation was not pronounced. It is suggested that the preference of glycine to other substrates oxidized in mitochondria is achieved in photosynthetic tissue by switching to rotenone-insensitive intramitochrondrial NADH oxidation and by increasing of alternative oxidase involvement in the presence of glycine.