Rendered protein by-products used as feed ingredients in companion animal, aquaculture, and livestock diets may be susceptible to protein oxidation during storage and processing. These ingredients are expected to be stable for up to 2 years at ambient temperatures, yet the stability and functionality of proteins in these ingredients during storage and processing is unclear. If these products are susceptible to protein oxidation, rendered proteins may contain a greater amount of oxidized products have the potential to induce changes in oxidative status when consumed by animals. Oxidative stress has clear implications on both livestock production and animal longevity. Consumption of oxidized lipids has been shown to induce oxidative stress and reduced performance in livestock. It is critical to understand if consumption of oxidized protein can also induce oxidative stress. The objective of this dissertation was to determine the extent to which storage and extrusion may impact protein oxidation in rendered protein meals, and how protein oxidation affects their suitability as an animal feed ingredient. It was hypothesized that storage and extrusion of rendered protein meals will cause an increase in protein oxidation that when fed to animals will reduce performance and induce oxidative stress. A study was conducted to determine if the common method to measure protein oxidation by carbonyls was more effective at detecting differences in protein oxidation than detecting specific oxidation products of lysine and arginine via mass spectroscopy analysis. It was determined that the mass spectroscopy approach was too specific and was not representative of total protein oxidation. Fish meal and poultry blood meal were shown to be susceptible to protein oxidation during 7 days of storage at 45C as well as during 6 months of storage at 20C. Mixed tocopherols were effective in limiting oxidation in fish meal stored at 45C for 7 days whereas ethoxyquin was more effective at limiting the extent of protein oxidation in fish meal stored long-term at 20C. The processing of these proteins into diets may also contribute to further oxidation. Protein meals were extruded, and carbonyls and other physical characteristics were evaluated on extrudate. Results from this study showed that extrusion induced protein oxidation in poultry blood meal, chicken by-product meal, and chicken by-product meal low ash. Mixed tocopherols were effective in limiting carbonyl formation in chicken by-product and chicken by-product low ash while ethoxyquin was effective at limiting carbonyl formation in fish meal. Physical characteristics were impacted by inclusion of antioxidant, but these differences were dependent on product. The final studies included in this dissertation aimed to determine if short-term consumption of oxidized rendered meals (spray dried plasma – 10% inclusion in the final diet, or chicken by product - 23% inclusion in the final diet) would induce oxidative status in a pig model. However, this research did not find that oxidized rendered meals induced oxidative stress in pigs when included in the diet for less than 35 days. The digestibility of protein was reduced in the diets that included oxidized rendered meal. An increase in crypt depth or villi height was also observed in both studies. A decreased availability in nutrients may be a factor in the decreased feed efficiency of pigs consuming the oxidized ingredient. It is still unclear how diets extremely high in rendered protein meals (30- 50%), fed the entire lifespan of an animal can impact oxidative status. Protein oxidation during storage and processing impacted many of the common rendered feed ingredients in this study and may impact their functionality during processing. When fed to animals, the oxidation of rendered meals decreased digestibility, altered gut morphology, and decreased growth efficiencies. However, relatively short-term feeding of these products did not induce systemic oxidative stress measured by protein, lipid, and DNA damage. Therefore, while rendered protein meals may increase in protein oxidation during storage and extrusion, the use of these ingredients does not induce oxidative stress during short term feeding. However, the reduced digestibility of protein should be considered when formulating diets with oxidized rendered protein meals.