Palavras-chave:Equino. Líquido sinovial. Hemoderivado. Estresse oxidativo. Plasma processado autólogo. As a cursorial animal, the horse relies on soundness to fully accomplish daily life activities and survival. Horses' natural ability for movement justifies breeding and training investments, and is explored to its maximum during athletic activities. Articular disorders, such as osteoarthritis and osteochondrosis, with their potential to reduce physical performance, are a therapeutic challenge. Autologous haemoderivates have appeared as safe and accessible options for the management of these conditions, but their widespread use has outpaced scientific evidence regarding mechanisms of action and ideal methods for preparation and administration. The antioxidant properties of autologous processed plasma, previously demonstrated in vitro on stimulated equine synovial fluid cells, posed a question on whether these effects would occur in vivo. Therefore, the present study aimed to verify autologous processed plasma´s effects in osteoarthritic and osteochondritic joints, when administered at the end of the arthroscopic procedure, with particular emphasis on possible antioxidant attributes. The study also aimed to detect evidences of oxidative stress in these disorders, what could further justify its use. For this purpose, synovial fluid was obtained immediately before the beginning of the surgery, and analyzed to yield an oxidant/ antioxidant, physical, cellular, molecular and inflammatory profile of osteoarthritis and osteochondrosis. After the end of the procedure, one group of horses received the intraarticular medication and another group of untreated horses served as controls. Forty-eight hours after surgery synovial fluid was retrieved from operated joints, and a second analysis was performed. Physical and cellular analyses were conducted immediately after sample collection and the supernatant was stored at -80º C for further evaluations. These consisted in catalase, superoxide dismutase, glutathione peroxidase, total antioxidant capacity, vitamin E, protein carbonyl groups, chondroitin sulphate, hyaluronic acid, prostaglandin E 2 , total protein and urea determinations.Osteoarthritic joints revealed greater activation of antioxidant defense systems, enzymatic and non-enzymatic, with greater activity of catalase, and greater concentrations of vitamin E, respectively, when compared to osteochondritic joints. Additionally, these joints had higher total nucleated cell counts and prostaglandin E 2 and total protein concentrations, revealing a more inflammatory profile when compared to that of osteochondritic joints. Nonetheless, in osteochondritic joints synovial fluid and matrix components degradation was more pronounced, as there were higher concentrations of chondroitin sulphate and hyaluronic acid in synovial fluid obtained from these joints. Results also showed the occurrence of oxidative stress, 48 hours after the arthroscopic procedure, through higher catalase activity in synovial fluid, reflecting activation ...