Sodium iothalamate produced a dose dependent increase in basal oxygen consumption when soybean lipoxygenase was incubated with arachidonic acid in 0.1M borate buffer pH 9.0. The increase in oxygen consumption was associated with an increase in absorbance at 234 nm indicating an increased conjugated diene formation. The stimulation of O2 consumption was demonstrated to be due to an increase in 1500H arachidonic formation. The increase in 1500H arachidonate formation could be blocked by mannitol which is an inhibitor of the lipoxygenase enzyme. N-methyl glucamine (meglumine) which is added to some preparations of iothalamate, was also able to suppress the increase in hydroperoxide formation in a dose dependent fashion.