Background: This study histopathologically investigated the local applicability of ketorolac tromethamine injected into knee joints of rats and its effects on cartilage and synovial tissue of the knees.Methods: The study involved 30 Sprague-Dowley rats with a weight of 250-300 gr. After halotan anesthesia, all the rats were injected 0.25 ml ketorolac tromethamine prepared with 0.9% serum physiologic at a total of 10 mg/ml into their right knee joints, comprising the study (ketorolac) groups.The left knee joints of all the rats were injected 0.25 ml 0.9 % serum physiologic and formed the control groups of the study. All the rats were sacrificed by phenobarbital in the following order: Group I (n=10) 24 hours after intra-articular (i.a.) injection, Group II (n=10) 48 hours after the injection, Group III (n=10) rats 5 days after the injection. The samples were then paraffin embedded and histopathologically examined. For inflammatory and degenerative changes, the parameters such as congestion, edema, fibrin exudation, synovial hyperplasia, neutrophil increase, macrophage increase, macrophage like cells (type A cells) and fibroblast like cells (type A cells) were studied.Results: In Ketorolac group, the rats which were sacrificed at the 24 th and 48 th hour had superficial erosion and decreased synoviocytes in the subsynovial layer of the inner surface of the joint capsule and synovial epithelium (P=0.127, P=0.042).Particularly, in Group III, which was sacrificed on the 5 th day, congestion, edema, neutrophile and macrophage increase in the subsynovial connective tissue, type B cell increase, and lymphocyte increase in the deeper sections of synovial membrane were detected in the joint capsules (P=0.000).None of the groups presented synovial membrane hyperplasia and fibrin exudation that could suggest severe inflammation.Conclusion: The long-term i.a. application of ketorolac should be avoided. On the other hand, since pure ketorolac is known to cause less inflammation than stabilized ketorolac, agents which will inhibit inflammation should be the choice of preference.