Abstract When intermediates of glucose metabolism such as glucose-6-phosphate, fructose-1,6-diphosphate and glyceraldehyde-3-phosphate are introduced microelectrophoretically into EL2 giants there is a diffuse increase in extramitochondrial (cytoplasmic and nuclear) pyridine nucleotide fluorescence, but little change in mitochondrial fluorescence. The response to phosphogluconate shows that the TPN-linked pentose phosphate pathway is operative in the EL2 giant in addition to the DPN-linked Embden Meyerhof pathway. In glucose-fed cells with a high redox potential of the extramitochondrial pyridine nucleotides, ADP increases the fluorescence and ATP has little effect, while in cells with low redox potential both ATP and ADP decrease the fluorescence. Cyclic AMP has no detectable effect. The response to G6P + ADP differs in starved, amytalized and glucose-treated cells. In the first two there is a consistent increase of extramitochondrial fluorescence, while in glucosetreated cells the response is correlated to the redox potential of the extramitochondrial pyridine nucleotides. With the combination FDP + ADP + Pi an increase of extramitochondrial fluorescence of comparable magnitude is recorded in both glucose-fed and starved cells. The mitochondria of starved cells respond by an active fluorescence increase to glutamate, malate, α-ketoglutarate and succinate + ATP. The mitochondrial response to glutamate can be controlled by microelectrophoretically introduced adenine nucleotides. In glucose-fed cells the mitochondrial response to glutamate is negligible, but there is an extramitochondrial increase of fluorescence. The fluorescence increase which follows addition of malate is of approximately comparable magnitude in the mitochondrial and extramitochondrial spaces.