The authors compared, in rat brain cortex slices, the oxidation of labelled glucose and acetate and the conversion of these precursors into amino acids during incubation in control salt-glucose medium and in medium with 47 mM K+, with the aim of determining with which of the two determinable tricarboxylate cycles raised oxygen consumption is associated in the presence of excess K+. Under the experimental conditions it was found that from U-[14C]-glucose more than double the amount of [14C]-CO2 was formed and that the rate of [14C] incorportation into the amino acids was likewise roughly doubled. This is indicative of activation of processes in the tricarboxylate cycle associated with the large glutamate pool. Incorporation from 1-[14C]-acetate into the total amino acids was not affected. Specific activity in glutamate and asparate was more than doubled, while glutamine specific activity fell to less than half. [14C]-CO2 production fell to 65%. This shows that the tricarboxylate cycle associated with the small glutamate pool, which is probably localized in the glia cells, did not participate in raised oxygen consumption in the presence of excess K+.