A 100 μM glutamate pulse administered to rat cerebellar granule cells causes a very rapid and progressive decrease in both cell and mitochondria! oxygen consumption caused by glucose and succinate addition, respectively. The respiratory control ratio, which reflects the ability of mitochondria to produce ATP, is reduced by 50% within the first 30 min after glutamate addition. Subsequent to glutamate exposure, a progressive decrease of respiratory control ratio to almost 1 was found within the following 3–5 h. The addition of extra calcium had no effect per se on oxygen consumption by cell homogenate.