Abstract A polarographic technique suitable for kinetic studies of either isolated or bound pyridine nucleotide-linked dehydrogenases was used to investigate the isocitrate dehydrogenases of yeast mitochondria. Intact mitochondria were found to contain both triphosphopyridine nucleotide- and diphosphopyridine nucleotide-linked isocitrate dehydrogenases with properties very similar to those which have been reported for the isolated dehydrogenases. The DPN-linked enzyme in situ exhibits a nonclassical, sigmoid substrate against activity relationship with respect to isocitrate and is activated by adenosine 5'-monophosphate. The TPN-linked enzyme in situ exhibits a hyperbolic substrate against activity relationship which is unaffected by AMP. The DPN-linked dehydrogenase also shows another non-classical increase in activity that occurs in response to high isocitrate concentrations or to heat treatment. This activation is characteristic only of untreated, intact mitochondria and may be secondary to an alteration of the mitochondria themselves.