To probe the interaction of zinc with polyunsaturated fatty acids we have studied the effect of zinc on the cooxygenation of ferrous iron and arachidonic acid. Zinc inhibited the process of cooxygenation in a concentration dependent fashion. Further evaluation of the interaction of zinc and arachidonic acid gave spectroscopic evidence that zinc, oxygen and arachidonic acid can form an unstable hydroperoxide-like complex similar to that postulated earlier for iron, oxygen and arachidonic acid. However, in the case of zinc the complex will not proceed further to form stable peroxides and the unstable complex falls apart to give zinc and arachidonic acid intact. The findings have implications for the role of zinc in enzyme reactions and for antioxidant reactions of zinc within the cell. The influence of zinc on platelet aggregation was also evaluated. Zinc was found to inhibit cell-cell aggregation. However, in contrast to the known ability of zinc to inhibit prostaglandin synthesis in a broken cell preparation, zinc did not inhibit prostaglandin or thromboxane synthesis in the intact platelet. Inhibition of platelet aggregation by zinc must result from some other action of this cation.