The effects of aqueous and vitreous humors and plasma on the rate of auto-oxidation of a rabbit brain homogenate were measured. Both aqueous and vitreous humors from normal eyes increased, while plasma decreased the rate of oxidation in the homogenate. During endotoxin-induced ocular inflammation the copper (Cu) and iron (Fe) concentrations of both the aqueous and vitreous humors increased, most likely due to the influx of their plasma binding proteins, ceruloplasmin (Cu) and transferrin (Fe). As both proteins are known to be antioxidants, it was not surprising to find that the aqueous and vitreous humor from the inflamed eyes had significant antioxidant activity. This antioxidant activity correlated well with the concentrations of Cu and Fe in aqueous humor and Cu but not Fe in the vitreous humor throughout the time course of the inflammatory response. Thus, entry of plasma proteins through disrupted blood ocular barriers may function in protecting ocular tissues against the increased oxidation which occurs during inflammation.