In pea leaf and potato mitochondria, external NADH oxidation was inhibited by concurrent oxidation of endogenous NADH and succinate. Succinate oxidation was also inhibited by concurrent oxidation of external NADH, but oxidation of endogenous NADH was not. NAD+-depletion studies suggested that glycine decarboxylase and other NAD-linked enzymes competed for available NAD+ within the matrix. However, at both high and low NAD+ levels, only the tricarboxylic acid cycle enzymes and malic enzyme were inhibited during concurrent oxidation with glycine. Measurements of the oxidation-reduction state of matrix NADH suggested that most of the mitochondria in the preparations contained both glycine decarboxylase and the tricarboxylic acid cycle enzymes and that steady-state levels of NADH were maximal with glycine alone as substrate. That is, there was no evidence for two populations of mitochondria being present. Nonetheless, malate stimulated state 4 and rotenone-inhibited O2 uptake in the presence of glycine. We conclude from these results that the priority glycine has as a substrate for leaf mitochondria is due to a priority that electrons from respiratory complex I have over those from complex II and the external NADH dehydrogenase, and the ability of glycine decarboxylase to compete favourably with tricarboxylic acid cycle enzymes for NAD+ in the matrix. Glycine may inhibit oxidation of other NAD-linked substrates by maintaining high matrix NADH/ NAD+ ratios. However, malate plus pyruvate appear to have access to some electron transport that is not accessible to glycine, at least under ADP-limiting conditions.