High-level inhibition of complex IV activity with KCN is required to increase the rate of glutamate release from depolarized synaptosomes. Synaptosomes (0.5 mg/ml) were preincubated at 37°C for 5 min before being depolarized with A 1 mM 4-aminopyridine or B 40 mM KCl. Rates of glutamate release at each concentration of KCN were plotted against percent inhibition of complex IV activity brought about by that concentration of KCN. Freehand curves were drawn through the results. Points shown represent the mean ± SEM for experiments carried out in triplicate on at least 3 separate synaptosomal preparations.