Time courses of incorporation of 13C from 13C-labelled glucose and/or acetate into the individual carbon atoms of amino acids, citrate and lactate in depolarized cerebral tissues were monitored by using 13C-n.m.r. spectroscopy. There was no change in the maximum percentage of 13C enrichments of the amino acids on depolarization, but the maxima were reached more rapidly, indicating that rates of metabolism in both glycolysis and the tricarboxylic acid cycle were accelerated. Although labelling of lactate and of citrate approached the theoretical maximum of 50%, labelling of the amino acids was always below 20%, suggesting that there is a metabolic pool or compartment that is inaccessible to exogenous substrates. Under resting conditions labelling of citrate and of glutamine from [1-13C]glucose was not detected, whereas both were labelled from [2-13C]acetate, which is considered to reflect glial metabolism. In contrast, considerable labelling of these two metabolites from [1-13C]glucose was observed in depolarized tissues, suggesting that the increased metabolism may be due to increased consumption of glucose by glial cells. The labelling patterns on depolarization from [1-13C]glucose alone and from both precursors [( 1-13C]glucose plus [2-13C]acetate) were similar, which also indicates that the changes are due to increased consumption of glucose rather than acetate.