This study investigates the impact of fish oil supplementation, a source of long chain polyunsaturated fatty acids (PUFA), for dogs with osteoarthritis (OA). Twenty-four dogs with OA from the Colorado State University Veterinary Teaching Hospital were randomly assigned to two groups, those receiving supplemental fish oil and a control group. They were fed with their respective diets for 63 days, and 5 plasma and synovial samples were obtained during the study to measure biochemical parameters of OA such as fatty acid composition, matrix metalloproteinases -2 and -9, tissue inhibitor of metalloproteinase –2), and bicyclo-PGE2, and significant differences in plasma and synovial n-3 and n-9 PUFA, as well as in MMP -2 and -9 and TIMP-2 were found.